ARTICLE pubs.acs.org/Macromolecules
Synthesis, Structure, and Properties of Alkyl-Substituted PPXs by Chemical Vapor Deposition for Stent Coatings Anna K. Bier, Michael Bognitzki, Jochen Mogk, and Andreas Greiner* Fachbereich Chemie und Wissenschaftliches Zentrum f€ur Materialwissenschaften, Philipps-Universit€at Marburg, Hans-Meerwein Strasse, D-35032 Marburg, Germany
bS Supporting Information ABSTRACT: High-molecular-weight un-cross-linked poly(p-xylylene) (PPX) derivatives (3) with one lateral linear alkyl substituent (methyloctyl) on each phenylene moiety were prepared by pyrolysis of corresponding paracyclophanes followed by chemical vapor deposition (CVD). Most of these PPX derivatives showed good solubility in organic solvents which allowed purification by reprecipitation, structural analysis by solution NMR techniques, and molecular weight analysis by gel permeation chromatography. High thermal stability of 3 was found by thermogravimetric analysis. The glass transition temperatures decreased systematically with increasing alkyl chain length from 53 to 23 °C. Increased elongation to break up to 380% was analyzed by stressstrain experiments for as-deposited films indicating excellent coating properties, which were proven by initial coating tests on stents.
’ INTRODUCTION PPXs are of fundamental as well technical interest where they are used as coating materials for electronic or medical devices like stents or pacemakers due to their excellent barrier and insulation properties, biocompatibility, and biostability.1 The combination of polymer synthesis and simultaneous film formation by vaporization, pyrolysis of [2.2]paracyclophanes, subsequent CVD, and polymerization of quinodimethanes under ambient conditions is unique.2 As a result, conformal pinhole free coatings on nearly any solid substrate can be prepared.1d PPXs have found numerous applications as stent coatings, including drug eluting stents.3 Although PPXs show very good performance as stent coatings, improvement of coating stability upon stent extension could be of major interest. The properties of PPX, including its thermal and mechanical properties are governed by the substitution pattern. Unsubstituted PPX is crystalline with melting under decomposition at 420 °C. PPX is transparent and insoluble in any solvent below 220 °C and shows an elongation to break of 1015%.1 Only a limited number of CVD-based PPX derivatives are known. For example, PPXs with chlorine and bromine substituents at the phenylene moieties are known, which are partially crystalline and fusible. The elongation to break of the dichloro-substituted PPX amounts to 220%.1 Only a few PPX derivatives with alkyl or phenyl substituents are known.4,5 Methyl- and ethyl-substituted PPX were obtained by CVD as hardly soluble polymers from the corresponding [2.2]paracylophanes. Poly(n-butyl-p-xylylene) was reported as a rubbery material with a crystallization point of 25 °C,6 which could make such PPX promising materials for stent coating applications. The overall expectation is that the flexibility of the coatings would increase with increase in alkyl chain length combined with a drop of glass transition r 2012 American Chemical Society
temperature and a loss in crystallinity. Increased solubility in organic solvents under ambient condition should allow structural proof by NMR and molecular weight and polydispersity analysis by GPC. Since the limited flexibility of stent coatings is a significant issue, it would be highly interesting to investigate alkyl-substituted PPX for such coating applications. The aim of this work is to elaborate a versatile and highly efficient route to poly(n-alkyl-p-xylylene)s and to investigate the role of alkyl chain length of the substituent on the property profile of PPX. Further scope of this contribution is to gain an understanding of the process for achieving high molecular weights obtained by the PPX CVD route, which is not yet well developed. The concept used for achieving the aims was to synthesize poly(n-alkyl-p-xylylene)s by CVD using the corresponding di(n-alkyl)[2.2]paracyclophanes and to use linear alkyl substituents which do not omit the volatility of the [2.2]paracyclophanes. On the basis of this, we want to suggest suitable novel PPXs for stent coating applications based on initial coating tests.
’ EXPERIMENTAL SECTION Materials. Diethyl ether was dried over phosphorus pentoxide; methanol (BASF), tetrahydrofuran (BASF), and chloroform (VWR) were distilled prior to use. 4,12-Dichloro[2.2]paracyclophane (1) was obtained by recrystallization of DPX C (Specialty Coating Systems) from 1,4-dioxane. Methyl iodide (Alfa Aesar, +98%), ethyl bromide (98%, Sigma-Aldrich), propyl bromide (99%, Sigma-Aldrich), butyl bromide (98%, VWR), pentyl bromide (99%, Sigma-Aldrich), hexyl bromide (99%, Sigma-Aldrich), heptyl bromide (99%, Sigma-Aldrich), Received: October 10, 2011 Revised: November 14, 2011 Published: January 17, 2012 1151
dx.doi.org/10.1021/ma202270w | Macromolecules 2012, 45, 1151–1157
Macromolecules octyl bromide (99%, Sigma-Aldrich), Ni(dppp)Cl2 (Sigma-Aldrich and VWR), and magnesium turnings (g99.9%, Carl Roth) were used as obtained. Instrumentation. GC/MS measurements were done with a QP5050 A instrument from Shimadzu with a 30 m FS-SE-54-CB-0.25 column, electron ionization unit, and helium as carrier gas. A program from 100 to 280 °C with a heating rate of 10 °C and an additional 20 min at 280 °C was chosen. The injection temperature was 300 °C, and the interface temperature was maintained at 230 °C. Elemental analysis was carried out on a Heraeus CHN-Rapid machine. 1 H (400 MHz), 13C (100 MHz), HSQC, COSY, and HMBC spectra were recorded on a Bruker DRX 400, and 1H (300 MHz) spectra were recorded on Bruker Advance 300 A at room temperature with CDCl3 as solvent. Thermogravimetric decomposition was analyzed by an 851 TG module from Mettler. In nitrogen atmosphere (flow rate: 50 mL/min) 1012 mg of the sample was placed in an alumina crucible, which was heated to 800 °C with a rate of 10 °C/min. For IR measurements an UMA 600 from Digilab with an ATR unit from Pike Miracle with diamond as top plate was used. Analysis of average molecular weight and molecular weight distribution were done by GPC with chloroform and THF as eluent. For each measurement 510 mg of polymer (directly after pyrolysis) was dissolved in 10 mL of chloroform or THF with toluene as internal standard. For chloroform GPC the flow rate was 0.5 mL/min, and the setup included a Knauer Smartline 1000 pump, three SDV columns (pore size 1000, 100 000, 1 000 000 Å) from PSS, and a Knauer refractive index detector (RI 2300). Calibration was done versus linear PMMA purchased from PSS. THF GPC was equipped with two SDV columns (10 μm, 60 0.8 mm2) from a PSS a triple detector array TDAmax 300 (RI was used); the working flow rate was 0.8 mL min1. Calibration was done versus polystyrene. For DSC an 821 DSC module from Mettler calibrated with indium and zinc standards were used. 1015 mg of the sample was placed into a sealed aluminum pan. In a nitrogen atmosphere (flow rate: 80 mL/min) a temperature program with a heating rate of 20 °C/min was applied. The glass transition temperature was taken as the inflection point of the observed shift of the baseline of the second heating cycle of the scan. UV/vis measurements were performed with a Lambda 35 from Perkin-Elmer precisely. WAXS was done with a powder diffractometer Panalytical X’Pert Pro PW3040/60 from Philips equipped with Cu Kα as a secondary monochromator. Mechanical measurements were performed by a Zwick Roell BTFR0.5TN.D14 equipped with a KAF-TC load sensor. The samples were prepared with a Rayran manual press with a dog-bone cutter ISO 5272-1BB. For static contact angle measurements the Contact Angle Measurement System G10 from Kr€uss equipped with a CCD Video Camera Module was used. For evaluation 1520 values where measured at different points of the sample surface. For SEM images a CAM Scan 4 with an acceleration voltage of 5 kV was used.
General Route for Preparation of 4,12-Di(n-alkyl)[2.2]paracyclophanes (2). 5.0 equiv of magnesium turnings was stirred under argon to activate the surface by mechanical abrasion of the magnesium oxide layer, and then diethyl ether was added. To the mixture were given to 5.0 equiv of the alkyl bromide (methyl iodide in the case of 4,12-dimethyl[2.2]paracyclophane) dropwise at 0 °C. In the following the reaction started and the alkyl bromide was diluted. When the reaction was finished the solution was stirred an additional hour at room temperature. The remaining magnesium residues were filtered off, and the Grignard solution was added dropwise to a suspension of 1.0 equiv of 1 and 5 mol % of [1,3bis(dipenylphosphino)propane]nickel(II) chloride Ni((dppp)Cl2) at 0 °C.
ARTICLE
Table 1. Yields, Elemental Analysis, and Melting Points of 1 and 2 elemental analysis calcd (found) no., formula
yield (%)
1, C16H14Cl2c
C (%)
H (%)
Tma (°C)
69.33 (69.35)
5.09 (5.08)
153
2a, C18H20
78
91.47 (91.20)
8.53 (8.58)
176b,d
2b, C20H24 2c, C22H28
83 86
90.85 (90.44) 90.35 (90.13)
9.15 (9.21) 9.65 (9.91)
155e 147
2d, C24H32
88
89.94 (89.70)
10.06 (10.07)
85
2e, C26H36
89
89.59 (88.89)
10.41 (10.40)
82
2f, C28H40
35
89.29 (89.21)
10.71 (10.72)
67
2g, C30H44
43
89.04 (88.69)
10.96 (10.95)
65
2h, C32H48
73
88.82 (88.32)
11.18 (11.08)
61
Determined by DSC. Heating rate: 10 K min1 for 2dh, 20 K min1 for 2b,c. b With optical microscope because vaporization starts at ∼150 °C. c Calcd: Cl, 25.58%. Found: Cl, 25.92%. d Literature value: 182183 °C. e Literature value: 152153 °C.5c a
Then the reaction mixture was heated under reflux until the conversion was completed (GC). After cooling to 0 °C the mixture was quenched with hydrochloric acid (2 M). The organic layer was separated, and the aqueous phase was extracted twice with diethyl ether. The combined organic layers were washed with saturated sodium chloride and sodium hydrogen carbonate solution. Then the organic phase was dried over magnesium sulfate, and the solvent was removed under vacuum. The raw product was recrystallized from methanol (2ch) or from ethanol (2a,b). The product was obtained as a colorless crystalline powder. Analytical data and yields of the synthesized 2 are reported in Table 1 and in the Supporting Information.
General Route for Preparation of Poly(n-alkyl-p-xylylene)s (3). For CVD process a self-constructed apparatus was used. It was composed of a tubular three zone oven (Pyrolus AT) with a total length of 115 cm and a diameter of 5.5 cm where in every zone a temperature between 25 and 1000 °C could be adjusted. Vacuum was generated with an Edwards S two-stage oil pump with base pressures between 2.0 103 and 3.0 103 mbar the pressure was controlled with an Edwards Pirani 1002 with a gauge PRL10 (DE0021-58-00). Between pump and deposition chamber three cooling traps were placed. CVD was done in a glass apparatus with vaporization chamber (l: 10 cm; d: 2.5 cm), a pyrolysis tube (fused quartz glass; l: 89 cm; d: 2.5 cm), and a deposition chamber with cooling jacket (l: 15 cm; d: 4.5 cm) maintained at 0 °C. Films of 3ac were prepared in a different chamber equipped with a cooled messing target (12.2 cm 4.1 cm) maintained at 0 °C inside the chamber perpendicular to the monomer flow, which was wrapped with aluminum foil to facilitate removal of the film. All glass parts were connected by standard ground joint (NS 29). For a CVD run 0.503.00 g of the precursor was placed in the vaporization chamber under vacuum, the oven was heated to a temperature between 500 and 640 °C in the pyrolysis zone, and the transportation zone was maintained at 300 °C. When the temperatures were adjusted and the pressure was constant, the vaporization unit was heated to temperatures between 90 and 185 °C until a slight rise in pressure ((0.10.2) 103 mbar) was observed. The temperature remained constant until the starting base pressure was reached again, and then the oven was cooled down; afterward, the polymer was brought into contact with air, and it was removed from the glass wall of aluminum foil by rinsing with distilled water. The polymer was dried at 40 °C for 24 h or without further purification used in GPC measurements (3ch). For all further measurements the polymer (3ch) was dissolved in chloroform and reprecipitated in methanol. Then it was dried for 3 days at 60 °C at a 1152
dx.doi.org/10.1021/ma202270w |Macromolecules 2012, 45, 1151–1157
Macromolecules
ARTICLE
Scheme 1. Synthesis of 2 in Kumada Coupling Reactions with 1 Used as Starting Material
Scheme 2. Preperation of 3 via Chemical Vapor Deposition
Table 2. Yield, Elemental Analysis, Contact Angle Measurements, and Results for TGA and DSC of 3 elemental analysis calcd (found) 3, molecular formula Tvap (°C) Tp (°C)
yield (%)
d
contact angle (deg)
C (%)
H (%)
thermal analysis T5% (%), Tmax (%) Tg (°C) Tm (°C)
a, (C9H10)n
87
600
quant
94 ( 3a
91.47 (91.25)
8.53 (8.57)
469, 491
53
245
b, (C10H12)n
110
640
quant
97 ( 2a
90.85 (90.71)
9.15 (9.18)
463, 489
17
e
c, (C11H14)n d, (C12H16)n
120 127
550 500
quant (90%c) 88c
99 ( 2a, 98 ( 2b 98 ( 3b
90.35 (90.19) 89.94 (89.66)
9.65 (9.73) 10.06 (10.04)
459, 488 444, 485
2 4
e 118
e, (C13H18)n
130
500
64c
99 ( 2b
c
89.59 (89.61)
10.41 (10.46)
439, 485
11
110
f, (C14H20)n
143
500
70
98 ( 3b
89.29 (89.33)
10.71 (10.72)
440, 487
21
121
g, (C15H22)n
155
500
64c
100 ( 2b
89.04 (89.21)
10.96 (10.88)
430, 486
26
120
h, (C16H24)n
185
500
44c
99 ( 2b
88.82 (89.29)
11.18 (11.02)
428, 484
29
117
Film surface as-deposited. b Solvent-casted film surface. c Yield after reprecipitation from MeOH. d Contact angle measurements were realized with a distilled water droplet volume between 10 and 15 μL. e In a temperature region between 25 and 400 °C no melting peak was observed (heating rate: 20 °C/min), Tp: pyrolysis temperature during CVD; T5%: thermogravimetric decomposition of 5% of the sample; Tmax: maximum of decomposition rate; Tg: glass transition temperature; Tm: melting peak. a
pressure of 15 mbar. Samples of 3ab and 3c prepared at 550 °C were used as deposited.
’ RESULTS AND DISCUSSION 4,12-Di(chloro)[2.2]paracyclophane (1) was isolated by recrystallization from DPX C, the commercial precursor of chlorine-substituted PPX. The synthesis of 4,12-di(n-alkyl)[2.2]paracyclophanes (2ah) as precursors for alkyl-substituted PPX (3) was performed in a one-step reaction by Kumada coupling of 1 and different alkylmagnesium bromides according to Scheme 1. Yields, elemental analysis data, melting points, and 1 H and 13C NMR data of 1 and 2 are given in Table 1 and the Supporting Information and compared well to literature data as far as available.5 The precursors 2ah were processed to 3ah according to Scheme 2 at different pyrolysis temperatures optimized for yield and a deposition temperature of 0 °C. 3ac were obtained as clear films without impurities, whereas polymers 3dh could only be obtained as opaque films. Thermogravimetric analysis and 1H NMR showed that 3dh were contaminated by low molecular weight fractions mainly precursor and cyclic trimer. However, due to the good solubility in organic solvents,
reprecipitation from chloroform/methanol allowed the purification of 3dh for further analysis. The yield after reprecipitation was between 44 and 90%, showing a decrease in yield for derivatives with longer alkyl chain length (Table 2). ATR-IR analysis of the polymers 3ah showed typical signals for sp3 CH stretching (2959, 2930, 2867 cm1), sp2 CH deformation (883, 819 cm1), and CdC ring stretching (1499, 1450 cm1) (Supporting Information). The chemical structure of 3bh was confirmed by NMR analysis (Supporting Information and Figure 1). 13C NMR spectra of 3bh reveal a “splitting” of the carbon signals (representative 3c, Figure 2) like it was also reported for 3b.7 It was expected that chains with head-to-tail as well as headto-head and tail-to-tail moieties would be formed, which can be well explained for substituted monomers as shown in Scheme 3. Similar findings were obtained with PPX derivatives prepared by the Gilch route and for the CVD route using esters of α,α0 -dihydroxy-pxylyenes as starting materials.4a,8 Two-dimensional NMR techniques were applied for further structural analysis with 3c as a representative example. 1H,1HCOSY showed the 3J-couplings (3JH1H2, 3JH2H4) from the side-chain atoms as expected; additionally, a 3J-coupling between H7 and H6 or H8 was visible, 1153
dx.doi.org/10.1021/ma202270w |Macromolecules 2012, 45, 1151–1157
Macromolecules which is important for the structure determination later on (Figure 3). The 1H,13C-HSQC (consistency) spectrum permitted clear assignments for 1JCH couplings, clarifying attachment of the same hydrogen group to the multiple carbon signals (Figure 4).
ARTICLE
The HMBC spectrum showed that the structure of the polymer was not totally resolved because the different connections of the monomer units did not allow an unambiguous assignment of
Figure 1. 1H NMR of 3c in CHCl3 showing the aromatic hydrogen atoms (68), the backbone CH2groups (3, 5), and the side-chain CaromCH2 (4), CH2 (2), and CH3 (1) and structure assignment referred to in all other spectra.
Figure 3. COSY spectrum of 3c in CDCl3 permitting the assignment of the side chain and especially important to show that hydrogen (6) or hydrogen (8) has no adjunct atom.
Figure 2. 13C NMR of 3c in CDCl3 with magnification (AE) showing splitting of carbon signals due to head-to-tail, head-to-head, and tail-totail structure of the polymer.
Figure 4. HSQC spectrum of 3c in CDCl3.
Scheme 3. Possible Polymer Structures Resulting from a substituted p-Xylylene Diradical
1154
dx.doi.org/10.1021/ma202270w |Macromolecules 2012, 45, 1151–1157
Macromolecules
ARTICLE
Figure 7. UV/vis spectra of 3ah in a region between 200 and 850 nm. The film thickness of deposited films (3ac) was 25 μm; film thickness of solvent-casted films 3dh was 2535 μm. Figure 5. HMBC spectrum of 3c in CDCl3.
Table 4. Comparison of Mechanical Data of 3 Table 3. GPC Results in Chloroform for Polymer Samples 3ch Prepared at 500 °C 3
Tp
Mn
Mw
Mz
PD
3 Tp, °C
c
500
227 000
598 000
1 174 000
2.6
ac
a
elongation to
Young’s
to break
break, % (av film
(lit.), GPa
modulus, GPa
(lit.), %
thickness, μm)
2.76 (CVDa)
2.51
230a
230 (20)
1.08
16b 275a
235 (25)
b
d
500
94 000
426 000
369 000
4.5
e f
500 500
258 000 152 000
950 000 400 000
2 190 000 839 000
3.5 2.6
bc
620
cc
550
0.42
380 (40)
3.9
cd
530
0.31
220 (177)
dd
525
0.16
300 (197)
ed
500
0.12
250 (171)
fd
500
0.06
290 (72)
gd hd
500 500
0.06 0.04
270 (42) 230 (36)
g h a
620
elongation Young’s modulus
500 500
12 2000 64 500
477 000 228 000
129 000 65 200
3.5
Sample was measured in THF, conventional calibration.
1.90 (Gilch ) 1.21 (CVDa)
a
3a: tensile strength, 9500 psi; tensile modulus, 400 000 psi; elongation to break, 230%. 3b: tensile strength, 11 000 psi; tensile modulus, 175 000 psi; elongation to break, 275%; 10% strain per minute.2 b 3a: tensile strength, 4600 psi; tensile modulus, 275 000; elongation to break, 16%.11b c As-deposited. d Solvent-casted.
Figure 6. Glass transition temperature of 3ah in dependence of the carbon chain length of the alkyl side chain.
C3 and C5 (CH2CH2 backbone) (Figure 5). Assignment was done for C6 by the missing coupling to H4, for H7 because of its coupling to quaternary C10, for C9 because of its coupling to H6 and H8, and for C11 because it is the only one coupling with H2 leading to the polymer assignments depicted in (Figure 1). Molecular weights of 3ch ranging from 230 000 to 950 000 were obtained by GPC (Table 3). Similarly, high molecular weights recently published by us were found for siloxane-
substituted PPX.9 The polydispersities of 3ch were between 2.6 and 4.5. Thermogravimetric analysis showed that the 3 were highly stable (T5%: 428469 °C; Tmax: 484491 °C) comparing well to commercial unsubstituted and chlorine substituted PPX (Table 2).4a,10 DSC showed a decrease in glass transition temperature (Tg) with increasing length of the alkyl substituents (Figure 6). For 3a a Tg of 53 °C and a Tm of 245 °C were found by DSC, which was in good agreement with literature (Tg: 5060 °C,2,11 Tm: 200 210 °C2,11a). For 3b a Tg of 17 °C was found which was also confirmed by literature (Tg = 25 °C2), but a Tm between 160 and 180 °C5c,6b could not be confirmed. For 3h a Tg of 29 °C was found. Good transparency of visible light of 3ac was confirmed by absorption spectroscopy. In contrast, 3dh appeared to be slightly opaque even after reprecipitation as already mentioned earlier, which was confirmed by absorption measurements (Figure 7). The crystallinity of as-deposited films of 3ac and solventcasted films of 3dh were studied by WAXS in dependence of 1155
dx.doi.org/10.1021/ma202270w |Macromolecules 2012, 45, 1151–1157
Macromolecules
ARTICLE
Figure 8. WAXS diagram of 3a and 3b as-deposited films stored at 40 °C for 24 h and 3c and 3h as solvent-casted film dried at 40 °C for 48 h. The annealing was done for 72 h with stepwise augmentation of temperature up to 200 °C.
Figure 9. SEM image of CVD-coated stainless steel stent model with 3c (film thickness: 3.1 μm) after expansion at different magnifications.
different annealing temperatures. In Figure 8 it is shown that the as-deposited films of 3ab treated at 40 °C showed distinct WAXS patterns, whereas 3ch showed a pronounced amorphous halo and broad peaks with less intensity. A possible explanation for this behavior is depicted in Scheme 3; multiple orientation possibilities of the alkyl chains in the ring have a great impact on the packing density of the polymer (e.g., favored parallel orientation of the phenylene units in the crystalline part of unsubstituted PPX could be disturbed).12 Upon temperature treatment crystallinity of 3a increased significantly by annealing up to 200 °C, which is still below the melting point (Tm: 245 °C) of the polymer. For 3b an increase in crystallinity was observed for annealing up to 150 °C. The other samples (3ch) showed a significant decrease in crystallinity (peak broadening and decreased peak intensity) upon annealing at 100 °C. Mechanical measurements of the PPX 3 showed a decrease of the Young’s modulus with increasing length of the alkyl side
chain (Table 4). CVD films of 3c showed the largest elongation to break with 380%, which is significantly higher as compared to PPX (1015%), whereas the Young’s modulus of 3c is only 0.4 GPa (PPX Young’s modulus = 2.41 GPa). For the solvent-casted films of 3dh the Young’s modulus was ranging from 0.04 to 0.31 GPa, and the elongation to break was between 230 and 300%. These promising values for the elongation to break were confirmed by CVD coating of stents by 3c followed by expansion of the stents. SEM images of the expanded stents showed conformal deposition of 3c on the stent surface and no visible delamination or damage of the coating (Figure 9). Contact angle measurement of as-deposited films 3ac and solution-cast films of 3ch showed contact angles against water ranging from 94° to 100°, which is not significantly higher than for unsubstituted PPX (85°) and chlorine-substituted PPX (90°) (Table 2). As far this difference in contact angle is relevant for cell 1156
dx.doi.org/10.1021/ma202270w |Macromolecules 2012, 45, 1151–1157
Macromolecules adhesion, which could play a major role in stent applications, will be the topic of an upcoming paper.
’ CONCLUSION The one-step synthesis using 4,12-dichloro[2.2]paracyclophane as starting material was proven to be a versatile route to 4,12-di(n-alkyl)[2.2]paracyclophanes as precursors for alkyl-substituted PPX. Poly(methyl-p-xylylene), poly(ethyl-p-xylylene), and poly(npropyl-p-xylylene) could be prepared in quantitative yields. [2.2]Paracyclophanes with longer alkyl substituents tolerated pyrolysis temperatures up to 500 °C. Molecular weights of 3 were high, showing that CVD polymerizations can yield high molecular weight of un-cross-linked polymers. The substitution pattern of 3 was governed by the formation of different isomeric repeating moieties and contributed to the lower crystallinity of 3 as compared to unsubstituted PPX. Glass transition temperature and crystallinity of 3 decreased with longer alkyl chain length. As a consequence, elongation to break was for all polymers >200% up to 380%, further indicating improved of mechanical properties. Initial tests on stents showed that the coatings with the alkyl-substituted PPX survive stent expansion which is an important but not the only criteria for applications on stents. Nevertheless, these results suggest alkylsubstituted PPX as promising candidates for demanding coating applications because they can be well characterized, and with this their properties can be, in contrast to present commercial PPXs, correlated with molecular parameters like the molecular weight and because their elongation to break is relatively high. The performance of this new alkyl PPX has to be verified for the demands of specific application, for example, for stent applications cell adhesion of alkyl PPX coatings, which is the topic of upcoming work.
ARTICLE
2006, 7, 2086–90. (c) Unger, F.; Westedt, U.; Hanefeld, P.; Wombacher, R.; Zimmermann, S.; Greiner, A.; Ausborn, M.; Kissel, T. J. Controlled Release 2007, 117, 312–321. (d) Wolf, K. V.; Zong, Z.; Meng, J.; Orana, A.; Rahbar, N.; Balss, K. M.; Papandreou, G.; Maryanoff, C. A.; Soboyejo, W. J. Biomed. Mater. Res., Part A 2008, 87, 272–281. (e) Hanefeld, P.; Agarwal, S.; Kumar, R. Macromol. Chem. Phys. 2009, 211, 265–269. (4) (a) Greiner, A.; Mang, S.; Sch€afer, O.; Simon, P. Acta Polym. 1997, 48, 1–15. (b) Sch€afer, O.; Brink-Spalink, F.; Smarsly, B.; Schmidt, C.; Wendorff, J. H.; Witt, C.; Kissel, T.; Greiner, A. Macromol. Chem. Phys. 1999, 200 (8), 1942–1949. (c) Ishaque, M.; Wombacher, R.; Wendorff, J. H.; Greiner, A. e-Polym. 2001, 5, 1–16. (d) Ishaque, M.; Greiner, A.; Agarwal, S. e-Polym. 2002, 31, 1–10. (5) (a) Gorham, W. F. Aklyated Di-p-xylylenes. Union Carbide Corp. U.S. Patent 3 117 168, 1964. (b) Yeh, Y. L. G.; William, F. J. Org. Chem. 1969, 34 (8), 2366–2370. (c) Ying, L. Yeh, Y. Process for the preperation of alkylated Di-p-Xylylenes. Union Carbide Corp. U.S. Patent 3 349 142, 1967. (6) (a) Gorham, W. F. Para-Xylylene Copolymers. Union Carbide Corp. U.S. Patent 3 288 728, 1966. (b) Gorham, W. F. Para-Xylylene Polymers. Union Carbide Corp. U.S. Patent 3 342 754, 1967. (7) Loy, D. A.; Assink, R. A.; Jamison, G. M.; McNamara, W. F.; Prabakar, S.; Schneider, D. A. Macromolecules 1995, 28, 5799–5803. (8) Simon, P.; Mang, S.; Hasenhindl, A.; Gronski, W.; Greiner, A. Macromolecules 1998, 31, 8775–8780. (9) Bier, A. K.; Bognitzki, M.; Schmidt, A.; Greiner, A.; Gallo, E.; Klack, P.; Schartel, B. Macromolecules 2011. (10) (a) Jellinek, H. H. G.; Lipovac, S. N. J. Polym. Sci., Part A-1 1970, 8 (9), 2517–2534. (b) Joesten, B. L. J. Appl. Polym. Sci. 1974, 18, 439–448. (11) (a) Szwarc, M. Polym. Eng. Sci. 1976, 16 (7), 473–479. (b) Gilch, H. G.; Wheelwright, W. L. J. Polym. Sci., Part A-1 1966, 4, 1337–1349. (12) Miller, K. J.; Hollinger, H. B.; Grebowicz, J.; Wunderlich, B. Macromolecules 1990, 23, 3855–3859.
’ ASSOCIATED CONTENT
bS
Supporting Information. Tables S1 and S2. This material is available free of charge via the Internet at http://pubs.acs.org.
’ AUTHOR INFORMATION Corresponding Author
*E-mail: greiner@staff.uni-marburg.de.
’ ACKNOWLEDGMENT The authors are indebted to Deutsche Forschungsgemeinschaft for financial support and to Specialty Coating Systems for the donation of DPX C. ’ REFERENCES (1) (a) Wolgemuth, L. The Surface Modification Properties of Parylene for Medical Applications. Buisinessbriefing: Medical Manufacturing and Technology 2002, 1–4. (b) Greiner, A. Poly(p-xylylene)s (Structure, Properties, and Applications). In The Polymeric Materials Encyclopedia; Salamone, J. C., Ed.; CRC Press: New York, 1996; Vol. 9, pp 71717180. (c) Alexandrova, L.; Vera-Graziano, R. Poly(p-xylylene)s, coatings and films. In Polymeric Materials Encyclopedia, 2nd ed.; Salamone, J. C., Ed.; CRC Press: New York, 1996; Vol. 9, pp 71807189. (d) Jamin, R. Innovation in Healthcare Delivery. Polymers for the Medical Industry 2001, Rapra Technology: Brussels, Belgium 2001, 17, 1–6. (2) Gorham, W. F. J. Polym. Sci., Part A-1: Polym. Chem. 1966, 4 (12), 3027–3039. (3) (a) Lahann, J.; Klee, D.; Pluester, W.; Hoecker, H. Biomaterials 2001, 22, 817–826. (b) Hanefeld, P.; Westedt, U.; Wombacher, R.; Kissel, T.; Schaper, A.; Wendorff, J. H.; Greiner, A. Biomacromolecules 1157
dx.doi.org/10.1021/ma202270w |Macromolecules 2012, 45, 1151–1157
