zyxwvu JOIJHNAL OF POLYMER SCIE'JCI3: PART A-1 VOL. 9, 2291-22911 (1971) zyxwvut zyx zyxw zyx zyxw Structure of Crystalline Polymers with Unbranched Long Side Chains* N. A. PLATfi, V. 1'. SHIBAEV, 13. S. PETHUI\HIN, YU. A. ZUBOV, and V. A. KARGIK, Chemistry Department, A t . V . Lomonosov Aloscow State University, Moscow, CJ.8.S.R. Synopsis The structure and thermodynamic properties of atactic and isotactic acrylic and methacrylic polymers containing 16-18 carbon atoms in the n-aliphatic side chains, and of ccpolymers of hexadecyl acrylate with isopropyl acrylate were studied by means of x-ray and differential thermal analysis. The crystallization of branched acrylic and methacrylic polymers and of acrylic copolymers proceeds in the form of a hexagonal crystal, regardless of the configuration of the backbone chain. Methods of ordering branched macromolecules are proposed, and the melting points, heats and entropies of fusion determined. The role of flexibility of the backbone chains in ordering and the crystallization processes was determined. I n the case of poly(n-alkyl acrylates) the backbone chain is involved in the crystalline lattice; this is not the case in methacrylates and copolymers of hexadecyl acrylate with isopropyl acrylate. Some similarity war assumed between the structure of biopolymers and synt.hetic branched polymers. INTRODUCTION Branched poly-a-olefins, poly(viny1 ethers) and poly(viny1 esters), poly(n-alkyl acrylates) and poly(n-alkyl methacrylates) are of interest due to their peculiar molecular structure. These polymers have two different structural units, (1) a backbone chain, and (2) long n-aliphatic side chains; the mobility of the units depends on the flexibility of the backbone chain and the type of attachment of the polymethylene branches to the main chain. The character of the macromolecular packing has to be determined by the mobility ratio between these unit8s. Previously we found'f2 that poly(n-alkyl acrylates) and poly(viny1 esters) having ten or more carbon atoms in the side chains crystallize with hexagonal unit cells, independent of their microtacticity. The cryst,allization of the polymers is governed by the hexagonal packing of the long methylene side chains. However, the influence of the chemical nature of the backbone chain on the structure of these polymers and the possibility of participation of the main chains in crystallization has not been clarified up to the present time. zyxwv * Presented a t the Inteniatioiial Symposium on Macromolecular Chemistry, held in Toronto, Canada, September 3-6,1968. 2291 0 1971 by John Wiley & Sons, Inc. 2292 zyxw zyxwv PLATE ET AL. In the present paper the cffccts of flexibility of the backbone chain and mobility of the methylene side chain (determined by the t.ype of junct'ion with the main chain) on t8hestructure :tnd thermodynamic properties of atactic and isotactic crystalllne polymers and copolymers having long side chains (such as polyacrylic, polymethacrylic and polyvinyl esters having 16-18 carbon atoms in the n-aliphatic side chain) is considered. The fusion temperatures (Tm), together with the heats ( A H ) and the entropies (AS) of fusion of these polymers have been determined. The structure of atactic and isotactic macromolecules with long side chains and their packing in the crystalline state have been studied. zyxw EXPERIMENTAL Synthesis of Monomers and Polymerization Procedure The polymers used were atactic poly(viny1 stearate) (PVSt-17), poly(nheptadecyl acrylate) and poly(n-octadecyl acrylates) (PA-17 and PA-18, respectively), atactic and isotactic poly(n-hexadecyl acrylate) (PA-16), poly(n-hexadecyl methacrylate) (PMA-16), as well as copolymers of nhexadecyl acrylate (A-16) with isopropyl acrylate (IPA). All the n-alkyl acrylate and n-alkyl methacrylate monomers used in this study were prepared by alcoholysis of methyl acrylate and methyl methacrylate, respectively, with the corresponding aliphatic long-chain alcohols; they were purified as in the previous w o k 1 Vinyl stearate was prepared by by acidolysis of vinyl acetate with stenric acid. Atactic poly(n-alkyl acrylates), poly(n-alkyl mcthacrylntes), and copolymers of A-16 with IPA were obtained by the photopolymerizntion of the corresponding monomers or their mixtures in chloroform solution in the presence of benzoyl peroxide a t room temperature. Vinyl stearate was bulk polymerized a t 70-80°C with benzoyl peroxide as n catalyst. Isotactic polymers and copolymers were prepared by anionic polymerization with butyllithium (0.003 g/ml of monomer) in toluene solution (10-15 wt-% based on monomer) a t -78°C. The conversion of the A-16-IPA copolymer ,was approximately 3-5%. All the polymers obtained were purified by repeated precipitation from toluene solution into methanol. The determination of the copolymer composition was performed by elemental analysis and infrared spectroscopy. Physical Chemical Investigation Poly (n-alkyl acrylates) and poly(n-alkyl methacrylates) were converted by hydrolysis into poly(acry1ic acid) and poly(methacry1ic acid), respectively. Poly(methacry1ic acid) was converted into poly(methy1 methacrylate) by treatment with diazomethane. The resulting poly(acry1ic acids) and poly(methy1 methacrylates) were used for estimation of the microt,:tcticity of the long-side-chain polymers, and the possibility of crystnlli- zy zyxwv zyxw zyx STRUCTURE OF CRYSTALLINE POLYMERS 2493 zation of poly(acry1ic acids) was studied (as described previously'). Highresolution NRIR spectra for poly(methy1 methacrylates) were obtained. Samples of polymers and copolymers having t,he same degree of poljm&zatfion ovcr the rmgc 2OOO-(iOOO were choson for the investig:~tionof the structure and thermodynamic properties. The crystallization of the polymers was carried out by cooling of the melt a t a rate of 1-2"C/min, accompanied by annealing a t different fixed temperatures below the melting point. X-ray photographs were taken with CuK a radiation, with the use of a flat-plate RKV-86 camera with variable specimen-to-film distances a t different temperatures. Long spacings were obtained by using a special lowangle camera. Differential thermal analysis (DTA) has carried out by using a Derivatograph apparatus (Hungary). The calibration of the Derivatograph for determination of the heats of fusion and melting was performed by use of a programmed heating rate of 0.3"C/min. Standards employed for the calibration were high purity stearic acid and eicosane. Heats of fusion were evaluated from the melting peak areas with an accuracy of f5 6 % . RESULTS AND DISCUSSION Earlier' we pointed out that crystallization of poly(n-hexadecyl acrylate) proceeds in the form of a hexagonal lattice. This structure is retained in similar polymers having a more rigid backbone chain and longer side chains. I n Table I interplanar spacings calculated from x-ray patterns of the oriented and unoriented polymers are given. Table I shows that all the polymers have the same interplanar spacings a t the wide angles corresponding to a hexagonal cell. For this type of packing, the predicted increment of long spacing for each additional carbon atom in the side chain of the monomer unit is 2.35 A. This value corresponds to half of the doubled step of a partially helical methylene chain and is in good agreement with the experimentally observed average increment of 2.5 A. This confirms the fact that in the polymers the side chains are extended a t right angles to the main chain. The x-ray patterns of the poly(alky1 acrylates) and PRIA-16 are characterized by several higher orders of reflection (2-5, Table I) of a very strong, sharp, low-angle reflection I. The orders of reflection are odd for poly(alky1 acrylates) and odd and even for PAIA-16 and copolymers of A-16 with IPA. These data are also in full agreement with the proposed structural models of macromolecules having long side chains. The existence of seven- or nine-order reflections in patterns of poly(alky1 acrylates) may be attributed t o a highly ordered arrangement of the backbone and side chains in the crystalline lattice, similar t o that of most b i o p ~ l y m e r s . ~Each polymer chain, in effect, forms a layer structure consisting of rigid "pivotsJJ (side chains) attached by different chemical links to the main chain and lined up in a regular manner. Such chains would be incorporated into the crystal by parallel packing, giving a three-dimensional layered network zyxwv zyxwvutsr zyxwvu zyx zyxwvuts zyxwv zy zyx zyxw TABLE I Interplanar Spacings of Polymers Having Long Side Chains d spacings, A Polymer PA-16, atactic and isotactic PA-17, atactic PA-18, atactic PMA-16, isotactic PMA-16, atactic A-16-IPA copolymer (56 mole-% A-16), atactic A - l e I P A copolymer (67 mole-% A-16), atactic Line 1 ( f lA) Line 2, (+0.2A) 42 44.5 47 27 29 26.5 14.7 14.9 15.8 13.5 14.7 13.9 26.5 13.6 Small angles -_ Line 3 Wide anglei: Line 4 (f0.06A) (=tO.O5A) 8.34 8.84 9.50 9.16 8.47 8.70 6.06 6.30 6.60 8.69 6.34 - Line 6 (+O.O4A) Line 70 (+0.02.%) Line (=kO.O2A) - 4.19 4.17 4.15 4.19 4.19 4.19 2.43 2.41 2.40 2.10 2.08 2.10 - 2.41 2.43 - 4.19 2.43 Line 5 o (k0.04A) - 5.40 - - 2 - - 8 5 ,-J 2 $ STRTJCTTJRE OF CRYSTALLIYE PO1,YUISRS 2295 zyxwvu zyxwvutsrq Fig. 1. Model of poly(hesadecy1 acrylate). Fig. 2. Model of poly(hexadecy1 methacrylate). The main chain seems to be involved in the crystalline lattice in the case of poly(n-alkyl acrylates) since reflection 1 corresponds to a length of two partially helical side chains plus the backbone chain. The situation is quite the opposite in the case of polymethacrylates and A-16-IPA copolymers. Their crystallization apparently proceeds without participation of the backbone chain. As it can be seen from Table I that PMA-16 and A-16IPA copolymers have a smaller long spacing d (reflection I), which corresponds to one layer packing of long side chains (Figs. 1and 2 ) . In the latter case the volume of the main chain is too large for the main chain to take part in crystallization. It can be theoretically calculated that for a structure of the type shown in Figure 1 only odd orders of reflection should be present; this actually was observed (Table I). For the packing shown in Figure 2, odd and even orders of reflection were observed (Table I).4 Naturally, one-layer packing gives a more defective crystalline structure. Hence the long spacing in PMA-16 and A-16-IPA copolymers is slightly zyx zyxwv zyx zyxwvu zyx P1,ATE ET AT,. 2296 TABLE I1 Temperature Dependence of Long Spacings of PA-16 and of PMA-16 Polymer Miornt,acticit,y of the main chain PA-16 Atactic PA-16 Isotsctic PMA-16 Atactic PMA-16 Isotactic Temperature Long T, of the of examination, spacing 4A polymer, "C 20 37 50-55 90 20 35 50-55 90 1.5 35 20 37 42 26 26 26 42 26.3 26.4 29 29 27 "C 38 36.5 22 26 zyxw zyx zyxwvu larger than one half of the long spacing in PA-16, and the orders of reflection are also correspondingly lower. The crystallization of poly(n-alkyl acrylates) with the main chain entering the crystal lattice occurs a t temperatures 10-15°C below the melt,ing points. At a temperature near T, the transition from two-layer to onelayer packing takes place. At temperatures higher than T,, the x-ray patterns of PA-16 and PMA-16 show only one strong reflection, corresponding to the long spacing. Table I1 shows that the characteristic long spacing of a layer is preserved a t temperatures much higher than the melting points, especially for atactic polymers. Disordering of isotactic polymers takes place more easily, and this seems to be connected with the influence of the backbone chain configuration. Isotactic PA-16 still shows a higher order reflection a t temperatures 15-20°C higher than T,, while isotactic PMA-16 under these conditions is disordered due to the rigidity of the main chain. The influence of main-chain rigidity can also be seen when polymers and copolymers are stretched. The stretching of poly(alky1 acrylates) , regardless of length of the side chain, causes splitting of diffractional lines and results in the appearance of six reflections corresponding to hexagonal packing (Fig. 3). These reflections are absent in x-ray patterns of oriented PMA-16 and A-16-IPA copolymers, these being characterized by a strong meridional reflection a t d = 4.19 A (Fig. 4). The theoretical treatment of this pattern indicates that this reflection, as expected, arises from hexagonally packed side chains having limited mobility due to higher rigidity of the backbone chain. A-16-IPA copolymers, regardless of stereoregularity of the backbone chains, are crystallized in a hexagonal lattice over a wide range of composition. Their melting temperatures are only slightly lower than that of PA-16. A limiting value of T , (17-18°C) has been found for atactic and isotactic copolymers of the same compositions, and this value corresponds to zyxwvu S'I'RUCTURE OF CRYSTALLINE POLYMERS 2297 IGg. 3. X-Ray diagram of oriented poly(octadecy1 acrylatc). Fig. 4. X-Ray diagram of oriented poly(hexadecy1 methacrylat,e). the melting points of n-paraffins having the same length as the long side chains of the copolymers. The data also confirm that in this case crystallization of the copolymers takes place due to the presence of long side chains; the copolymers are crystalline up to a composition with 80 mole-yo content of IPA. zyxw zyxwvut TABLE I11 Thermodynamic Properties fo Polymers Having Long Side Chains Polymer or monomer hlicrotacticity of polymer PA-16 PA-16 A-16 A-16 PA- 17 A-17 PA-18 A-18 A-18 PMA-16 I'hl A- 16 LTA-16 PVSt-17 VSt-17 Isotact,ic Atactic Atactic Atactic - Isotactic Atactic Atact,ic - Trn ( + 0 . 5 ) , AH,, Type of Cell "C calk AS,, eu Hexagonal Hexagonal Hexagonal Tricliriic Hexagonal Hexagonal Hexagonal Hexagonal Triclinic Hexagonal Hexagorid Triclinic Hexagonal Orthorhombic 36.5 38.0 20.0 26.0 46.0 28.0 49.0 31.0 34.0 26.0 22.0 18.8 19.9 21.5 42.4 24.3 26.6 24.6 27.7 43.7 11.4 9.4 34.0 19.8 37.9 18.0 19.0 21.7 42.0 23.6 27.4 24.8 29.5 48.3 11.8 9.9 36.0 19.0 35.3 20.0 49.5 34.0 zyxw zyxw zyxw PLATE ET AL. 2298 zyx zyxwv On the basis of x-ray and therrnographic analysis it was concluded that the monomers of the studied polymers can exist in various crystalline modifications; one is hexagonal (Table 111), and the others triclinic or orthorhombic. This is established by comparison of interplanar spacings of monomers with those of paraffin^.^ The difference in T, of acrylates and polyacrylates crystallized in hexagonal form equal to about 16-18 results from variations of enthalpy and entropy of fusion due to the decrease of mobility of monomer units attached to the macromolecular chain. The values of enthalpy and entropy of fusion of polyacrylates are close to those for hexagonal packed monomers. These values increase with the number of carbon atoms in the side chain. The lower T , of PM-4-16 in comparison with that of PA-16 can be explained by the considerably smaller values of AH, and AS,, due to steric hindrance brought about by one-layer type packing of the rigid macromolecules of PMA-16. The influence of the type of the branch junctions to the backbone chain is illustrated by a comparison of PA-17 and PVSt-17, the latter being more rigid. A comparison of the thermodynamic properties of isotactic and atactic PA-16 and PMA-16 revealed some differences, namely, higher rigidity of isotactic polyacrylate chains than of atactic chains, and also lower rigidity of isotactic polymethacrylate backbone chains than of the corresponding atactic chains. Hence the influence of configuration of the main chain on such properties, which is very pronounced for atactic and isotactic polymers having short side chains, is negligible in our case owing to the structural similarity of these systems. zyxwvut zyxwv zyxwv References 1. V. P. Shibaev, B. S. Petrukhin, Yu. A. Zubov, N. A. Platk, and V. A. Kargin, V y s o k m l . Soedin., lOA,216 (1968). 2. N. A. PlatA, V. P. Shibaev, B. S. Petrukhin, and V. A. Kargin, in Macromolecular Chemistry, Tokyo-Japan, 1966 ( J . Polym. Sci.C, 23), I. Sakurada and S. Okamura, Eds., Interscience, New York, 1968, p. 37. 3. B. K. Vaineschtein, X-Ray Diffraction of Macromolecules, Academy of Sciences USSR, Moscow, 1963. 4. Yu. A. Zubov, R. S. Petrukhin, and V. P. Hhibaev, Vysokmol. Soedin., 12B. 290 (1970). .5. B. S. Petrukhiu, V. 1’. Shibaev, m d N. A. Plrttd, Vpokomol. Soedin., 12A, 687 (1970). Received +January7, 1971 View publication stats