Synthesis, Properties, and Processing of New Siloxane-SubstitutedPoly(p-xylylene) via CVDAnna K. Bier, Michael Bognitzki, Alexander Schmidt, and Andreas Greiner*

Fachbereich Chemie and Wissenschaftliches Zentrum fur Materialwissenschaften, Philipps-Universitat Marburg,Hans-Meerwein Strasse, D-35032, Marburg, Germany

Emanuela Gallo, Patrick Klack, and Bernhard Schartel

6.35 Flammschutz von Polymeren, BAM Federal Institute for Materials Research and Testing, Unter den Eichen 87, 12205 Berlin,Germany

ABSTRACT: The synthesis of a disiloxane-functionalized[2.2]paracyclophane and its polymerization to the correspond-ing siloxane-substituted poly(p-xylylene) via chemical vapordeposition (CVD) has been described. Because of the en-hanced solubility of the siloxane substituted poly(p-xylylene)analysis of the molecular structure by NMR, molecular weight,and polydispersity by gel permeation chromatography (GPC),and processing by film casting as well as nanofiber formationby electrospinning was possible. Structural isomers were found by NMR which was expected due to the isomeric mixture of theprecursor. High molecular weights at moderate polydispersities were found by GPC which was unexpected for a vapor phasedeposition polymerization. The amorphous morphology in combination with a low glass transition temperature led to highelongation at break for the siloxane substituted poly(p-xylylene). Significant difference for the wetting versus water was found foras-deposited films, solution cast films, and nanofibers obtained by electrospinning with contact angles up to 135° close tosuperhydrophobic behavior.

1. INTRODUCTIONPoly(p-xylylene)s (PPXs) form an important class of polymerswhich is in use as barrier coatings for packaging, medical,automotive, aerospace, and electronic applications.1 The mostimportant features are their biocompatibility, excellentinsulation properties (in terms of the high dielectric constant),low dissipation factor, and high chemical and thermal stability,including excellent moisture barrier properties.2

In the technical process PPXs (trade name parylene) areobtained via chemical vapor deposition (CVD) using the so-called Gorham process.3 The vaporization of [2.2]para-cyclophanes, followed by pyrolysis of these precursors at temp-eratures between 500 and 700 °C under reduced pressure yieldsquinodimethanes, which polymerize spontaneously on nearlyany solid substrate at ambient temperatures.4 The final productof this process is a highly conformal pinhole free coating(coating thickness >0.7 μm5) of PPX on the given substrate.The main advantages of this process are solvent and initiator-free products, no side products, quantitative yields, and mildreaction temperatures.6,7 Although four precursors arecommercially available the overall number of suitableprecursors is limited and thereby the full potential of thisinteresting class of polymers with its unique combined processof polymerization and film formation cannot be fully exploited.Major limitations for new precursors are their volatility and thethermal stability of attached substituents.8 PPX derivatives from

[2.2]paracyclophanes equipped with iodine, bromine, ethyl,aminomethyl, cyano groups or different degrees of fluorinationattracted some attention but were never commercialized.9

In the past few years, mainly surface techniques provided aneasily applicable tool for changing the properties of inert PPXfilms. Plasma or (photo)chemical treatment hydrophilized thePPX surface by not well-defined functionalization, and thesetreatments were often accompanied by a loss of the char-acteristic optical and mechanical properties of the PPXs.10

Grafting techniques offer another easy access to surfacefunctionalization but all benefits of the CVD process are lostby usage of catalysts and solvents.11 Copolymerization of dif-ferent functionalized [2.2]paracyclophanes or other vinyliccopolymers led to either inhomogenous films, undesired sidereactions, or high contents of starting materials present in theobtained films.12−18

For example film compositions using a monofunctionalized[2.2]paracyclophane can be changed by temperature con-trolled deposition, even a separated deposition of completelyfunctionalized and unfunctionalized PPX is possible.3,13

Pyrolysis of monofunctionalized [2.2]paracyclophanes oftendoes not lead to desired compositions of 50:50 e. g. for

Received: September 21, 2011Revised: November 27, 2011Published: December 27, 2011

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poly(alkinyl-p-xylylene-co-p-xylylene) a composition of 80 to 20for functionalized to unfunctionalized units was found.14,15

Easily different gradient copolymers can be formed16 but acopolymerization basically faces the same problems known formonofunctionalized [2.2]paracyclophanes used as startingmaterials in the CVD process because at least two differentquinodimethanes (reactive species) with different depositionproperties are present in the gaseous phase.Using acrylic comonomers in CVD often leads to high

amounts of unreacted acrylates deposited as liquid in thedeposition chamber which have to be removed by annealing.17

Also the proper control of the film composition is hardlyachieved17 or a gradient film can be formed instead.18 To avoidthese problems often droplets of acrylic comonomer (low vaporpressure) are placed in the deposition chamber and a PPX filmis polymerized on this substrate to form a surface functionalizedfilm.19

Recently, the synthesis of various monofunctionalized[2.2]paracyclophanes has been successfully accomplished.Subsequent CVD led to a reactive coating material to whichbiomolecules or dyes could be attached.16,20 In these casescomposition and homogeneity of the obtained film were oftennot well-defined or differed from the expected composi-tion.14,15,21 Another tool for changing the surface propertieswas the growth of PPX nanorods by setting an angle of 15°(oblique angle) between the monomer flow and the substrate,which has been further developed to a powerful tool forengineered PPX films with unidirectional wetting proper-ties.22,23 The field could surely be further developed withadvanced PPX derivatives by CVD and by better character-ization of molecular properties like molecular configuration,molecular weight, and polydispersity. Improved mechanicalproperties would be of importance for numerous coatingapplications typically for PPX coatings, e.g., stent coatings.We wondered, whether the mechanical properties with

respect to a drastic increase in strain at break could be achievedby bulky substituents at the PPX backbone. Our concept was touse siloxane substituents at the phenylene moieties, whichotherwise also provide high inertness to bases and acids,insulation, and high thermal stability. Furthermore, it isexpected, that siloxane substituents would hydrophobizePPX which has been shown previously for other materials,e.g., inorganic surfaces.24 Another important outcome ofsuch bulky substituents could be enhanced solubility of theresulting PPX derivatives, which could give chance formolecular weight analysis by GPC, which has not been doneto for CVD-based PPX to the best of our knowledge.Although film formation indicate enhanced molecular weightof Gorham-type PPX the unusual polymerization of a solid-state type polymerization in combination with vapor deposi-tion of a gaseous monomer raises interesting questionswith respect to molecular chain growth, chain length, andmolecular chain length distribution.

2. EXPERIMENTAL PART2.1. Materials. Methyltriphenylphosphonium bromide (Fisher

Scientific, 98%), sodium hydrogen carbonate (Fisher Scientific) magne-sium sulfate (Acros, 97%), n-butyllithium (Sigma-Aldrich, 2.5 M solu-tion in hexane), s-butyllithium (Sigma-Aldrich, 1.4 M solution incyclohexane), propylene carbonate (Sigma-Aldrich, 99%), hydrochloricacid 37% (VWR, AnalaR Normapur), xylene (VWR, isomeric mixture,98%), (Pt(0)-1,3-divinyl-1,1,3,3-tetramethylsiloxane complex (Kar-stedt catalyst) (Heraeus GmbH, 20% Pt in xylene), sodium chloride(Carl Roth), chloroform-d1 (CDCl3) (C. Roth, 99.8 atom % D) were

used as obtained. Tri-o-tolylphosphine was synthesized accordingto the literature.25 THF, DMF, diethyl ether, and cyclohexane(BASF) were dried over phosphorus pentoxide and distilled priorto use. Hexane, toluene, and ethanol (BASF) were distilled priorto use.

2.2. Analytical Techniques. 1H (400 MHz), 13C (100 MHz),and 29Si NMR (100 MHz), 1H,13C-HSQC, and 1H,1H−COSY spectrawere recorded on a Bruker DRX 400 or Avance 300 A, respectively, atroom temperature with CDCl3 as solvent.

GPC analysis was performed with 5−10 mg of polymer (directlyafter pyrolysis) in 10 mL chloroform with toluene as the internalstandard. The flow rate was 0.5 mL/min, and the setup included aKnauer Smartline 1000 pump, three SDV columns (pore size 1000;100 000; 1 000 000 Å) from PSS and a Knauer Refractive IndexDetector (RI 2300). Calibration was performed by using linearpolystyrene purchased from PSS.

GC/MS measurements were done with a QP5050 A instrumentfrom Shimadzu with a 30 m FS-SE-54-CB-0.25 column, electronionization unit and helium as carrier gas. A program from 100 to 280 °Cwith a heating rate of 10 °C and an additional 20 min at 280 °C waschosen. The injection temperature was 300 °C and the interface tem-perature was maintained at 230 °C.

Mechanical properties were determined using a Zwick Roell BT-FR0.5TN.D14 equipped with a KAF-TC load sensor. The sampleswere prepared with a Rayran manual press using a dogbone cutter ISO5272−1BB. For cyclic measurements, grip-to-grip separation of 20 mm,test speed of 10 mm/min, and preload of 0.1 N was used. Elongationwas 100% for each of the 25 cycles with 20 s between them. Forelongation to break, a grip-to-grip separation of 20 mm, test speed of25 mm/min, a starting speed of 1 mm/min and a preload of 0.1 Nwere used.

For differential scanning calorimetry (DSC), a 821 DSC modulefrom Mettler calibrated with indium and zinc standards was used.Then, 10−15 mg of the sample was placed into a sealed aluminumpan and heated/cooled under nitrogen with a heating/cooling rateof 20 K/min. The glass transition temperature was taken at theinflection point of the observed shift of the baseline of the secondheating run.

Thermogravimetric analysis was performed by means of a 851TG module from Mettler under nitrogen atmosphere (flow rate:50 mL/min), 10−12 mg of the sample was placed in an aluminacrucible which was heated to 800 °C at a rate of 10 °C/min. Thermo-gravimetric analysis under synthetic air or nitrogen respectively with aflow of 30 mL/min and a temperature range between 25 and 900 °Cwere accomplished with a TGA 209 F1 from Netsch coupled with anIR Tensor 27 (4000−600 cm−1) from Bruker and a mass spectrometerQMS 403C Aeolos from Netzsch (range: 0 to 140 m/z). In an aluminacrucible 10 mg of the polymer was heated at a rate of 10 °C/min.

For static contact angle measurements the contact angle measure-ment System G10 from Kruss equipped with a CCD video cameramodule was used. For evaluation, 5−10 values were measured atdifferent points of the sample surface.

Electrospinning of a 2.9 wt % solution of siloxane modified PPX,pyrolized at 500 °C in chloroform, led, due to the low conductivity, tofiber production with a narrow spinning area. The addition of 5 wt %of benzyltributylammonium bromide led to a broader spinning area.The best results were obtained using a high voltage (15 to 20 KV) ofthe cathode and with no voltage applied to the collector electrode atan electrode distance of 15 cm. The temperature was 18 °C, and theair humidity was 45%.

For IR measurements, an UMA 600 from Digilab with an ATR unitfrom Pike Miracle with diamond as the top plate was used.

SEM images were taken on a 7500F SEM from Jeol.2.3. Precursor Synthesis. Synthesis of 4,12-Diformyl[2.2]-

paracyclophane (2). A 2 L three-necked, round-bottom flaskequipped with a dropping funnel and an argon inlet was filled with46.13 g (126.00 mmol, 1.00 equiv) of 1 dissolved in 1.2 L of THF.The solution was stirred under argon at −65 °C then 300 mL(420.00 mmol, 3.33 equiv) of s-butyllithum solution (1.4 M) incyclohexane was added dropwise. The solution immediately turned

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orange. After stirring for 2 h at −65 °C, 48.5 mL (630 mmol, 15 equiv)of dimethylformamide were added dropwise, and the solution wasstirred without cooling for 1 h. The solution was washed three timeswith 400 mL of brine, twice with 400 mL of saturated sodiumhydrogen carbonate solution and dried over magnesium sulfate. Thesolvent was removed under reduced pressure at 60 °C. The rawproduct was recrystallized from toluene to yield 26.64 g (108.00 mmol,80%) of colorless crystals.

1H NMR (300 MHz, CDCl3): δ (ppm) = 9.93 (s, 2H), 7.04 (d, 2H,J = 2.0 Hz), 6.62 (2H, dd, J = 2.0 Hz, J = 7.8 Hz), 6.52 (d, 2H, J = 7.8Hz), 4.17−4.08 (m, 2H), 3.32−3.24 (m, 2H), 3.19−3.10 (m, 2H),3.05−2.96 (m, 2H).

13C NMR (75 MHz, CDCl3): δ (ppm) = 191.9, 142.9, 140.5, 136.9,136.5, 135.2, 34.3, 32.8.Mp: 166 °C.IR: ν (cm−1) = 3019, 2930, 2855, 2750, 1672, 1589, 1552, 1485,

1399, 1277, 1219, 1183, 1134, 946, 868, 791, 721, 651, 618.MS (EI, m/e): M+ = 264, M+/2 = 132, 99%.Synthesis of 4,12-Divinyl[2.2]paracyclophane (3). In a three

necked, round-bottom flask equipped with dropping funnel, coolerand an argon inlet, a suspension of 88.38 g (247.4 mmol, 3.8 equiv) ofmethyltriphenylphosphonium bromide in 1.2 L of THF was cooled to0 °C. Then 100 mL (250.0 mmol, 3.8 equiv) of n-butyllithium inhexane was added dropwise, and the temperature was maintainedbetween 0 and 5 °C. After 2 h an orange solution was obtained and thesolid was nearly dissolved. Then 17.19 g (65.10 mmol, 1.00 equiv) of2 was added in portions, and the temperature increased slightly(3−5 °C). At room temperature, the solution was stirred for 2 h. Thenthe reaction was stopped by adding 500 mL of hydrochloric acid (5%)dropwise. The organic phase was separated, and the aqueous phasewas diluted with brine and extracted with three portions of THF. Thecombined organic layers were washed with a saturated sodium chloridesolution and a saturated sodium hydrogen carbonate solution.Afterward, the solution was dried over magnesium sulfate and thesolvent was completely removed under reduced pressure at 60 °C. Thesolid containing phosphonium salt was extracted with hexane (4 times600 mL). After evaporation of the solvent, the raw product wasrecrystallized from ethanol to yield 13.73 g (52.73 mmol, 81%) ofcolorless crystals.

1H NMR (300 MHz, CDCl3): δ (ppm) = 6.83−6.62 (m, 6H),3.37 (dd, 2H, J = 7.7 Hz, J = 1.8 Hz), 5.57 (dd, 2H, J = 17.3 Hz, J =1.3 Hz), 5.29 (dd, 2H, J = 10.8 Hz, J = 1.3 Hz), 3.46−3.34 (m, 2H),3.16−3.06 (m, 2H), 3.00−2.90 (m, 2H), 2.83−2.72 (m, 2H).

13C NMR (75 MHz, CDCl3): δ (ppm) = 139.4, 137.7, 137.5, 135.3,133.4, 130.1, 129.3, 114.3, 34.2, 33.0.Mp: 178 °C.IR: ν (cm−1) = 3041, 2936, 2891, 2852, 1896, 1580, 1533, 1474,

1450, 1389, 1188, 1028, 827, 646.MS (EI, m/e): M+ = 260, M+/2 = 130, 99%.Synthesis of the Siloxane-Modified [2.2]Paracyclophane (4). A

suspension of 4.00 g (15.38 mmol) of 3 in toluene, cyclohexane andpentamethyldisiloxane was added with 5 mL of a solution of 4.2 mL ofKarstedt-catalyst in 22 mL of propylene carbonate. The light yellowsuspension turned bright orange and then yellow again after 10 min, asthe solid dissolved slowly. A GC/MS measurement showedquantitative conversion. The product was obtained as black oil, andpropylene carbonate was removed via steam-distillation. The obtainedblack oil was purified by column chromatography (hexane:Et2O, 10:1)to yield 7.53 g (85%) of colorless oil, which crystallized slowly withina few days. The product contained three different constitutional iso-mers with amounts of 56% (4a), 35% (4b), and 9% (4c), respectively(Scheme 1).

1H NMR (300 MHz, CDCl3): δ (ppm) = 6.59−6.56 (m, 0.8H),6.49−6.44 (m, 1H), 6.37−6.24 (m, 2.2H), 6.17−6.07 (m, 2H), 3.42−3.27 (m, 2H), 3.09−2.93 (m, 4.1H), 2.83−2.59 (m, 3.4H), 2.39−2.25(m, 2H), 1.43−1.40 (m, 2.0H), 0.77−0.64 (m, 2.5H), 0.17-(−0.15)(m, 30H).

13C NMR (75 MHz, CDCl3): δ (ppm) = 144.4, 142.4, 139.7, 139.3,139.1, 138.9, 136.7, 136.7, 136.5, 133.8, 133.7, 133.7, 133.6, 133.5,133.4, 133.0, 129.6, 129.1, 127.6, 127.6, 126.3, 126.2, 124.9, 34.0, 34.0,

33.9, 33.6, 33.5, 32.9, 27.8, 27.8, 27.0, 19.1, 19.0, 12.5, 12.4, 2.1, 1.8,0.3, 0.3, −1.6, −1.9.

29Si NMR (CDCl3): δ (ppm) = 7.12, 7.10, 5.60.MS (EI, m/e): M+ = 557 (M+), 263 (Me3SiOSiMe2C8H7

+), 147(Me3SiOMe2

+), 133 (Me3SiOSiMe2+).

Mp: 42 °C.IR: ν (cm−1) = 2951, 2901, 2857, 1589, 1438, 1408, 1251, 1173,

1048, 867, 833, 802, 779, 752, 684.Anal. Calcd for C30H52Si4O2: C, 64.68; H, 9.41; Si, 20.17. Found: C,

64.45; H, 9.46; Si, 20.57.2.4. Polymer Synthesis (5). The siloxane-containing polymer

was obtained via CVD in a custom built apparatus with 3 heatingzones, each with a diameter of 5.5 cm and a length of 35 cm. Theworking pressure was between 1.8 and 2.2 × 10−3 mbar. Precursor 4(0.50 g) was sublimed at 135 °C (heating zone 1) and the vapor wasled through a quartz glass tube maintained between 420 and 580 °C(heating zone 2) where the corresponding quinodimethanes formed,heating zone 3 was set to 300 °C. In the deposition chamber, aborosilicate glass chamber with a cooling jacket, the reactive monomergas polymerized spontaneously to form the desired siloxane containingpolymer on the chamber walls, which were maintained at 0 °C. ForGPC measurements the polymer was used without further purification.For thermo analysis, mechanical and NMR measurements the polymerwas dissolved in chloroform, precipitated in methanol and then driedfor 48 h at 15 mbar and 60 °C.

2.5. Characterization of 5e. Yield: 100% substance in depositionchamber, 69% after reprecipitation in methanol

IR: ν (cm−1) = 2955 (m), 1497 (w), 1447 (w), 1413 (w), 1252 (s),1169 (w), 1047 (s), 833 (s), 802 (s), 785 (m), 687 (w), 631 (w).

1H NMR (300 MHz, CDCl3): δ (ppm) = 7.36−6.64 (m, 3H),3.05−2.41 (m, 5.6H), 1.37 (bs, 0.9H), 0.89 (bs, 1.3H), 0.29 to −0.17(m, 15H).

13C NMR (100 MHz, CDCl3): δ (ppm) = 143.4, 142.9, 140.1,136.89, 129.2, 129.0, 128.6, 127.2, 125.8, 124.4, 38.0, 37.8, 35.6, 34.9,34.1, 26.2, 20.5, 15.9, 2.1, 1.9, 0.3, −0.9.

T5% = 444 °C, Tg = −10.4 °C, Mn = 620 000, Mp = 1 176 000, Mw =1 314 000, D = 2.11.

Scheme 1. Synthesis of the Siloxane-Modified[2.2]Paracyclophane 4 Starting with 4,12-Dibromo[2.2]paracyclophane 1

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Anal. Calcd (5f): C, 64.68; H, 9.41; Si, 20.17. Found: C, 64.02; H,9.15; Si, 19.93; N, 0.12.

3. RESULTS AND DISCUSSION3.1. Synthesis of the Siloxane-Modified [2.2]Para-

cyclophane. The precursor 4 was synthesized by a three-stepprocedure according to Scheme 1. 4,12-Dibromo[2.2]para-cyclophane 1 was reacted with s-BuLi and DMF to form thecorresponding 4,12-diformyl[2.2]paracyclophane 2, which wasconverted to 4,12-dinvinyl[2.2]paracyclophane 3 by the Wittigreaction. The siloxane substituted precursor 4 was obtained asa product mixture (isomers 4a, 4b, 4c) by hydrosilylationreaction.3.2. Polymer Synthesis. The siloxane-substituted PPX 5

was synthesized according to the Gorham procedure bypyrolysis of 4 (isomeric mixture) at different pyrolysistemperatures (Scheme 2). In contrast to most of the known

PPXs obtained by the Gorham process, 5 showed excellentsolubility in organic solvents like toluene, THF, andchloroform, which allowed work-up by reprecipitation andanalysis in solution. The yield after pyrolysis was nearlyquantitative, but work-up of the raw product by reprecipi-tation showed a maximum yield for 5e at 500 °C (Figure 1).

All further analytical data, with the exception of the GPCanalysis, refer to 5e. Analysis of the filtrate of 5e by MALDI−TOF proved that the raw product was mainly contaminatedby unreacted 4 and cyclic trimer of 5. The IR spectra of5e showed strong signals at 1250 cm−1 (Si−CH3) and1050 cm−1 (Si−O−Si) indicating siloxane moieties, whichwas confirmed also by 29Si NMR spectra showing signals at7.75, 7.56, and 6.84 ppm (Figure 2). 1H and 13C NMRspectra of 5e display the substitution pattern of the isomericmixture of 4 (Figure 3, 4). The broad 1H NMR signals andthe multiple 13C NMR signals also indicate a head-to-head, ahead-to-tail and a tail-to-tail connection within the polymer.

1H,1H−COSY and HSQC NMR were employed for furtherstructural characterization and showed the presence ofsegment A and segment B deriving from the isomeric mixtureof the precursor (Figure 5, 6).5 showed very high molecular weights with relatively

polydispersities between 1.3 and 3.0 (Table 1). The peakmolecular weight went through a maximum in correlation withthe pyrolysis temperature, where 5e showed the highest and 5i

Scheme 2. Synthesis of Siloxane-Modified PPX 5 by CVD atVarious Temperatures

Figure 1. Yield of 5a−i depending on different deposition temper-atures Tp between 420 and 580 °C directly after pyrolysis and afterwork up procedure (dissolving in chloroform and reprecipitation inmethanol).

Figure 2. 29Si NMR spectra of 5d, 5e, and 5h.

Figure 3. 1H NMR spectrum (300 MHz, CDCl3) of 5e. Numbersshow hydrogen signals belonging to both possible structure elements(A, B). Letters show signals only belonging to one structure element.

Figure 4. 13C NMR spectrum (100 MHz, CDCl3) of 5e.

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the lowest peak molecular weight. The GPC traces of 5e and 5ishowed slight shoulders (Figure 7).

The physical appearance of 5e was like a soft colorlessamorphous material, which was confirmed by DSC measurementwhere only a glass transition temperature (Tg) was detected at−10 °C. In comparison, the monochloro-substituted PPX is acrystalline polymer with a Tg of 80 °C.26 No crystalline meltingtemperature was detected by DSC prior to thermaldecomposition, which was found to be by TGA above400 °C with a 5% weight loss at 442 °C. The IR spectrum ofthe degradation products showed typical signals at 3800 and2364 cm−1, which indicate water and carbon dioxide, and thepresence of silicon is verified by Si−H, Si−CH3, and Si−O−Sivibration (2128, 1257, 1061, and 694 cm−1). Signals at 3015,2960, 1600, 914, and 845 cm−1 reveal the presence of aromaticand aliphatic moieties (Figure 8). The mass spectrum alsoshowed the presence of water (m/z 18) and CO2 (m/z 22and 44) as well as ion series for [CnH2n+3Si]

+ with m/z 45, 59,and 73 and [CnH2n+3SiO]

+ with m/z 75 and 89 indicatingthe cleavage of the Si−O−Si bond and formation of tri-alkylsilanol and trialkylsilane groups during decomposition.Also characteristic signals for aliphatic groups (m/z 39, 52) aswell as benzene and benzyl groups (m/z 78, 103, 105, 117,133) were detected. On the basis of these results the de-gradation model depicted in Scheme 3 is suggested. Accordingto the bond dissociation energy values, the Si−C bond with76 kcal/mol was the first bond cleaved leading to alkyldisiloxane groups. Then a cleavage of the backbone C−C bond(83 kcal/mol) led to a formation of benzene and benzyl fragments,and cleavage of the strongest Si−O bond (110 kcal/mol) led totrialkylsilane and trialkylsilanol groups.

Figure 5. 1H,1H−COSY spectrum of 5e.

Figure 6. 1H,13C-HSQC spectrum of 5e.

Table 1. GPC Results of 5b−i Including Number Averaged Molecular Weight (Mn), Weight-Averaged Molecular Weight (Mw),Molecular Weight at Peak Top (Mp), and Polydispersity (PD)

sample Tpyrolysis/°C pressure/mbar Mn/Da Mw/Da Mp PD

b 440 2.4 × 10−3 393 000 602 000 698 000 1.53c 460 3.2 × 10−3 453 000 696 000 734 000 1.54d 480 3.2 × 10−3 801 000 1 243 000 1 292 000 1.29e 500 2.2 × 10−3 622 000 1 314 000 1 176 000 2.11f 520 3.4 × 10−3 508 000 715 000 716 000 1.41g 540 1.8 × 10−3 389 000 704 000 740 000 1.81h 560 1.8 × 10−3 385 000 555 000 578 000 1.44i 580 1.8 × 10−3 233 000 711 000 440 000 3.04

Figure 7. GPC traces of 5 pyrolized at different temperatures between440 and 580 °C. The calibration curve was obtained by retention timesof polystyrene standards with toluene as the internal standard.

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The soft nature of 5e was confirmed by stress−strainexperiments at 20 °C, which showed an average Young’smodulus of 0.02 GPa (Table 2). Compared to technically used

monochloro-substituted PPX with an elongation at break of200% the average elongation at break of 5e is 470%.25 Cyclicmeasurements showed that the polymer remained elongated by10% after elongation to 100% but in the following cycles thepolymer nearly maintained its shape (Figure 9).Significant differences in the wetting behavior of water on

different surfaces of 5e were found by contact angle measure-ments. The as-deposited film of 5e (135°, attention: con-taminated by precursor and trimer) as well as the electrospunsample (135°) of 5e showed significantly larger contact anglesof water as compared to the solution cast film (103°) (Figure 10).The difference in contact angle is most likely due to morestructured surfaces of the as-deposited film and the electrospunsample (Figure 11). For comparison unsubstituted PPX (81°)and monochloro-substituted PPX (90°) have significantly lowercontact angles,9d but structured PPX films showed exceptionalwetting behavior.22,23

Figure 8. Release of gaseous products analyzed by IR and MS from 5e maintained at different temperatures.

Scheme 3. Decomposition Model and Mass Fragments for Siloxane-Modified Polymer 5e from Degradation Studies

Table 2. Average Values for Mechanical Measurements of 5e

Young'smodulus/GPa

maximum force/MPa

elongation atbreak/%

average value 0.0202 32.0 469standarddeviation/%

16.7 13.6 7.5

Figure 9. Cyclic stress−strain tests of 5e, which was elongated todouble its size 25 times with a rate of 25 mm/min and a break of 20 sbetween each cycle.

Figure 10. Contact angles on different surface morphologies of 5e:(A) film as-deposited, (B) solvent-casted film, and (C) electrospunfiber mat.

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4. CONCLUSIONS

PPX with siloxane substituents can be prepared by thetechnically well established Gorham process with [2.2]para-cyclophanes as precursors. In contrast to other PPXs obtainedby the Gorham process the siloxane substituted PPX 5 issoluble in organic solvents at ambient temperatures, whichallowed analysis by NMR techniques and by GPC. Very highmolecular weights were found for 5, which is surprising as thereaction is heterogeneous by vapor deposition of the monomeron a solid polymer. The solubility of 5 and the relatively lowpolydispersities also indicate no cross-linkings and, if at all, alow degree long chain branching, which was a matter of debatefor many decades.Not unexpectedly, 5 is an amorphous polymer with a low

glass transition temperature, which is most likely due to thebulky and flexible siloxane substituents and the isomeric sub-stitution pattern of the precursor. Somewhat surprisingly, thecontact angle did not increase as expected from siloxane sub-stituents but enhanced hydrophobicity could be found withelectrospun nanofiber surfaces of 5e. Nevertheless, 5e andrelated PPX derivatives could show interesting wetting behaviorwith structured PPX films similar to the work of Demirel et al.by shadowing growth mentioned before.

■ AUTHOR INFORMATION

Corresponding Author*E-mail: greiner@staff.uni-marburg.de.

■ ACKNOWLEDGMENTS

The authors thank Deutsche Forschungsgemeinschaft forfinancial support and Specialty Coating Systems for [2.2]paracyclophane (Parylene N).

■ REFERENCES(1) Xylylene Polymers. Kirk-Othmer Encyclopedia of ChemicalTechnology [Online]; Wiley & Sons: Posted March 14, 2008.

http://onl inel ibrary .wi ley.com/doi/10.1002/0471238961.2425122502050103.a01.pub2/ababstra (accessed May, 2011).(2) Greiner, A.; Mang, S.; Schafer, O.; Simon, P. Acta Polym. 1997,48, 1−15.(3) Gorham, W. F. J. Polym. Sci., Part A: Polym. Chem. 1966, 4 (12),3027−3039.(4) Wintermantel, E.; Ha, S.-W. Medizintechnik mit biokompatiblenWerkstoffen und Verfahren; 3rd ed.; Springer: Berlin/Heidelberg,Germany, 2002.(5) Hanefeld, P.; Sittner, F.; Ensinger, W.; Greiner, A. e-Polym. 2006,26, 1−6.(6) Simon, P.; Mang, S.; Hasenhindl, A.; Gronski, W.; Greiner, A.Macromolecules 1998, 31, 8775−8780.(7) Alf, M. E.; Asatekin, A.; Barr, M. C.; Baxamusa, S. H.; Chelawat,H.; Ozaydin-Ince, G.; Petruczok, C. D.; Sreenivasan, R.; Tenhaeff,W. E.; Trujillo, N. J.; Vaddiraju, S.; Xu, J.; Gleason, K. K. Adv. Mater.2010, 22 (18), 1993−2027.(8) Fink, J. K. High Performance Polymers; William Andrew: Norwich,2008.(9) Santayana, G. WBF Draft 1999, 1863−1952.(10) Senkevich, J. J.; Yang, G.-R.; Lu, T.-M. Colloids Surf., A 2003,216 (1−3), 167−173. Meng, E.; Li, P.-Y.; Tai, Y.-C. J. Micromech.Microeng. 2008, 18 (4), 1−13. Herrera-Alonso, M.; McCarthy, T. J.Langmuir 2004, 20 (21), 9184−9189. Pruden, K. G.; Sinclair, K.;Beaudoin, S. J. Polym. Sci., Part A: Polym. Chem. 2003, 41, 1486−1496.(11) Wahjudi, P. N.; Oh, J. H.; Salman, S. O.; Seabold, J. A.; Rodger,D. C.; Tai, Y.-C.; Thompson, M. E. J. Biomed. Mater. Res., Part A 2009,89A (1), 206−214. Lahann, J.; Langer, R. Macromol. Rapid Commun.2001, 22 (12), 968−971. Jiang, X.; Chen, H.-Y.; Galvan, G.; Yoshida,M.; Lahann. J. Adv. Funct. Mater. 2008, 18 (1), 27−35.(12) Pu, H.; Wang, Y.; Yang, Z. Mater. Lett. 2007, 61 (13), 2718−2722.(13) Gorham, W. F. Para-Xylylene Copolymers. U.S. Patent 3,288,728,Nov. 24, 1966.(14) Senkevich, J. J.; Woods, B. W.; McMahon, J. J.; Wang, P. I.Chem. Vap. Deposition 2007, 13 (1), 55−59.(15) Lahann, J.; Klee, D.; Pluester, W.; Hoecker, H. Biomat. 2001, 22,817−826. Lahann, J.; Klee, D.; Hocker, H. Macromol. Rapid Commun.1998, 19 (9), 441−444. Elkasabi, Y.; Yoshida, M.; Nandivada, H.;Chen, H.-Y.; Lahann, J.Macromol. Rapid Commun. 2008, 29, 855−870.(16) Elkasabi, Y.; Lahann. J. Macromol. Rapid Commun. 2009, 30 (1),57−63. Chen, H.-Y.; Lahann, J. Langmuir 2011, 27 (1), 34−48.(17) Gaynor, J. F. Polymeric Thin Films by Chemical Vapor Depositionfor the Microelectronics Industry. Ph.D. Thesis, Virginia PolytechnicInstitute and State University: Blacksburg, VA, October 1995.(18) Gaynor, J. F.; Desu, S. B.; Senkevich, Jay. J. Macromolecules1995, 28, 7343−7348.(19) Bobrowski, M.; Skurski, P.; Freza, S. Chem. Phys. 2011, 382 (1),20−26. Naddaka, M.; Asen, F.; Freza, S.; Bobrowski, M.; Skurski, P.;Laux, E.; Charmet, J.; Keppner, H.; Bauer, M.; Lellouche, J.-P. J. Polym.Sci., Part A: Polym. Chem. 2011, 49 (13), 2952−2958.(20) Elkasabi, Y.; Lahann. J. Methods Mol. Biol. 2011, 671, 261−279.Chen, H.-Y.; Lahann, J. Langmuir 2011, 27 (1), 34−48.(21) Elkasabi, Y.; Lahann, J.; Krebsbach, P. H. Biomaterials 2011, 32(7), 1809−1815. Senkevich, J. J.; Woods, B. W.; McMahon, J. J.;Wang, P. I. Chem. Vap. Deposition 2007, 13 (1), 55−59. Lahann, J.;Klee, D.; Hocker, H. Macromol. Rapid Commun. 1998, 19 (9), 441−444.(22) Demirel, M. C.; Boduroglu, S.; Cetinkaya, M.; Lakhtakia, A.Langmuir 2007, 23 (11), 5861−5863. Cetinkaya, M.; Boduroglu, S.;Demirel, M. C. Polymer 2007, 48, 4130−4134.(23) Malvadkar, N. A.; Hancock, M. J.; Sekerogly, K.; Dressick, W. J.;Demirel, M. C. Nat. Mater. 2010, 9, 1023−1028.(24) Heck, R. F.; Ziegler, C. B. J. Org. Chem. 1978, 43, 2941−2946.(25) Krumpfer, J. W.; McCarthy, T. J. Langmuir 2011, 27, 11514−11519.(26) Greiner, A. Poly(p-xylylene)s (Structure, Properties, andApplications). In The Polymeric Materials Encyclopedia; Salamone, J.C., Ed.; CRC Press: New York, 1996; Vol. 9, pp 7171−7180.

Figure 11. (A and B) SEM images of electrospun 5e obtained fromCH3Cl (2.9 w%): solution A, magnification 3.3000; solution B,magnification 10.000. (C) SEM image with magnification of 3.000,film surface 5e deposited horizontally to the monomer flow at 0 °Cdirectly after pyrolysis. (D) SEM image with magnification of 3.000,film surface of solvent casted-film of 5e.

Macromolecules Article

dx.doi.org/10.1021/ma2021369 | Macromolecules 2012, 45, 633−639639