Structural change in the Brill transition of Nylon m/n (2) conformational disordering as viewed from the temperature-dependent infrared spectral measurements

The structural changes in the Brill transition of aliphatic Nylons m/n have been investigated by carrying out the temperature-dependent infrared spectral measurement for Nylon 6/10 -[-NH-(CH₂)₆-NHCO-(CH₂)₈-CO-]_n-, Nylon 6/12 -[-NH-(CH₂)₆-NHCO-(CH₂)₁₀-CO-]_n- and Nylon 10/10 -[-NH-(CH₂)₁₀-NHCO-(CH₂)₈-CO-]_n-. They took the crystal structure of α-form at room temperature. By heating the samples up to the Brill transition temperature region, the conformation of the methylene segments was found to change remarkably from the all trans-zigzag form to the disordered conformation constructed by shorter trans-zigzag segments combined with some gauche bonds. At the same time, the twisting motion was found to occur around the CH₂-amide bonds. During this order-to-disorder transition of molecular conformation, the intermolecular hydrogen bonds were kept alive although they were weaker gradually with increasing temperature. The methylene segments sandwiched by NH groups [NH(CH₂)_mNH] were found to be disordered more remarkably than those of CO(CH₂)_nCO sequence due to the difference in torsional energy barriers around the CH₂-NH and CH₂-CO bonds.     

Normal mode analysis of syndiotactic polypropylene (T2G2T6G2) form

Syndiotactic polypropylene of all-trans form is transformed to (T₂G₂T₆G₂) form (new form) on exposing to xylene vapor under tension. This new form was characterized by infrared spectra and normal mode analysis. By increasing the gauche component from the all-trans to the new form, the infrared intensities of some bands increase. The potential energy distributions (PED) and L vectors for six monomers, which compose the repeating unit, were investigated. As a result, the PED and L vectors of the conformation-sensitive bands were found to be localized at the gauche position.     

Molecular dynamics simulation of the structural and mechanical property changes in the Brill transition of nylon 10/10 crystal

In order to confirm the structural features in the Brill transition of aliphatic nylons, molecular dynamics calculation has been performed at the various temperatures for nylon 10/10 crystal as a model. With an increase in temperature, the torsional motions around the methylene-amide and methylene-methylene bonds were activated. These motions became more remarkable above 420 K and the interconversion between trans and gauche forms occurred frequently, resulting in the disordered conformation of the methylene sequences and the pseudo-hexagonal packing of these parts. During such a drastic structural disordering, however, the hydrogen bonds were kept alive although the bond strength became weaker. These calculations were found to be consistent with the experimental results by the temperature-dependent X-ray diffraction and infrared spectral measurements. The Young's modulus along the chain axis was also calculated as a function of temperature: it decreased remarkably from 250 GPa at 0 K to 180 GPa at 300 K due to a small contraction of the skeletal chain by only about 0.2-0.5% through the torsional motion of the skeletal chains, and the modulus decreased furthermore to 80 GPa at 550 K because of the larger conformational disordering of the skeletal chains. The Young's modulus in the direction perpendicular to the chain axis was also found to decrease remarkably in parallel to the change of the chain packing mode.     

Microscopically-viewed relationship between the chain conformation and ultimate Young's modulus of a series of arylate polyesters with long methylene segments

Relationship between the chain conformation in the crystal lattice and the ultimate Young's modulus has been discussed on the basis of the crystal structural information revealed by the X-ray diffraction analysis for a series of arylate polyesters with long methylene segments (–[–COC₆H₄CO–O(CH₂)_mO–]_n–). The X-ray structural analysis revealed that the molecular chains take the all-trans-zigzag conformations for all of the even-numbered polyesters and their model compounds as well as the odd-numbered polyesters with the methylene segmental length longer than (CH₂)₁₄. These chain conformations have been correlated well to the ultimate Young's modulus along the chain axis or the crystallite modulus E_c, which has been estimated experimentally by the X-ray diffraction method under a constant stress and also predicted theoretically using the X-ray-analyzed crystal structures on the basis of the molecular mechanics method. The E_c was found to show the minimum at around m = 4–6 and increased gradually with an increment of m and approached the crystallite modulus of polyethylene, 235 GPa (X-ray value) ∼ 316 GPa (calculate) at an infinite m value. This behavior of E_c as a function of the number of methylene segmental units m was reasonably interpreted by developing the theoretical equation of E_c for a simplified zigzag chain model composed of a repetition of two linear rods representing the benzene–ester and methylene segmental parts respectively. These findings may promise that the mechanical property of arylate polyester can be controlled by adjusting the methylene segmental length m.     

Structure of polyurethane elastomers. X-ray diffraction and conformational analysis of MDI-propandiol and MDI-ethylene glycol hard segments

X-ray diffraction and conformational analysis have been used to investigate the structure of polyurethane hard segments prepared from diphenylmethane 4,4′-diisocyanate (MDI) with propandiol (PDO) and ethylene glycol (EDO) as chain extenders. The results are compared with those obtained previously for MDI-butandiol (BDO) hard segments. In the latter system, the poly(MDI-BDO) chains are fully extended with an all-trans conformation for the O(CH₂)₄O unit, and a monomer repeat of 18.95 Å. The unit cell is triclinic with a tilted base plane such that adjacent chains are staggered along the fibre axis. In contrast both poly(MDI-PDO) and poly(MDI-EDO) adopt unstaggered structures, i.e. the chains are in register and the unit cell base planes are perpendicular to the chain axis. The monomer repeats of 16.2 Å for poly(MDI-PDO) and 15.0 Å for poly(MDI-EDO) are shorter than the predicted repeats for fully-extended chains, indicating that these polymers have contracted conformations containing some gauche CH₂ groups. Conformational analysis shows that the 16.2 Å repeat for poly(MDI-PDO) can be achieved with the O(CH₂)₃O unit in the trans-gauche⁺-gauche⁺-trans or gauche⁺-trans-trans-gauche⁺ conformations. Similarly the 15.0 Å repeat for poly(MDI-EDO) is predicted for the gauche⁺-trans-gauche^? conformation for the O(CH₂)₂O unit. These conformations are of higher energy than the all-trans fully-extended chains. This may explain the higher crystalline perfection of the poly(MDI-BDO) hard segments, for which crystallization in the all-trans form will probably provide a greater driving force for phase separation.     

Studying the length of trans conformational sequences in polyethylene using Raman spectroscopy: a computational study

The long standing question of how long a trans sequence in a polyethylene needs to be in order to contribute to the all-trans Raman bands at 1060 and 1130 cm⁻¹ is addressed. Various hexadecane, C₁₆H₃₄, conformers were studied using ab initio type calculations revealing vibrational frequencies and intensities. We found that only trans sequences longer than 10 trans bonds reveal Raman intensity primarily selectively at the same frequencies as the C-C stretching modes arising from crystalline polyethylene. Since it is statistically highly unlikely that such long trans sequences occur in the amorphous phase, intensity arising from all-trans sequences not belonging to crystalline structure may only be expected as a consequence of strong non-equilibrium conditions (e.g. shear), or the presence of tie-molecules.     

Conformational disorder in the Brill transition of uniaxially-oriented nylon 10/10 sample investigated through the temperature-dependent measurement of X-ray fiber diagram

X-ray fiber diagram of uniaxially-oriented nylon 10/10 sample has been measured at the various temperatures. The 11th layer line originating from the methylene zigzag sequences became diffuse above the Brill transition region (140-170 °C), while the layer reflections corresponding mainly to the original repeating period of the parent nylon chains were kept almost unchanged. This observation makes us confirm that in the Brill transition region, the conformational disordering occurs in the methylene sequential parts with keeping the repeating period of the whole skeletal chain by still forming the intermolecular hydrogen bonds between the amide groups of the neighboring chains. This conclusion is consistent with the already-reported infrared spectral data. The characteristic X-ray diffraction patterns observed at the various temperatures were reproduced well by a computer simulation technique utilizing the previously-reported molecular dynamics calculation results.     

Structural information from progression bands in the FTIR spectra of long chain n-alkanes

The low temperature FTIR spectrum of long chain n-alkanes has been investigated in the region between the C-C stretching and CH₂ twisting fundamentals (1050-1133 cm⁻¹). With successive annealing and cooling stages, extended chain crystals of n-C₁₉₈H₃₉₈ show an improvement in the regularity of the progression bands observed. This is related to a ‘perfecting’ of the crystals. A once-folded sample of the same alkane shows additional features between 1050 and 1100 cm⁻¹, attributed to resonance modes from a tight (110) fold. These disappear on transformation to the extended form, to be replaced by progression bands. Assignment of the individual bands enables the length of the all-trans chain to be estimated and this method is used to show that centre-branched long chain n-alkanes have a folded conformation. It is also shown that the chain length derived from such FTIR data for a 1:1 molar mixture of n-C₁₆₂H₃₂₆ and n-C₂₄₆H₄₉₄ is consistent with a triple layer superlattice structure.     

Conformational studies of poly(N-methyl-l-alanine) by n.m.r. spectroscopy

Nuclear magnetic resonance (n.m.r.) and circular dichroism of $\text{poly}\text{(N-}\text{methyl-}\text{l}\text{-alanine}\text{)}$ in methylene $\text{chloride}\text{-d}{}_2\text{dichloroacetic acid}$ were investigated. In methylene chloride-d₂$\text{poly}\text{(N-}\text{methyl-}\text{l}\text{-alanine}\text{)}$ was found to consist of nearly all-trans amide bonds and assume a stable secondary structure. Trace amounts of cis amide bonds were also present. The addition of dichloroacetic acid destroyed the secondary structure and induced a drastic change of the n.m.r. spectrum, which was similar to that observed with the monomeric amide, $\text{N-}\text{acetyl}\text{-N-}\text{methyl-}\text{l}\text{-alanine}$ dimethylamide. In comparison of the polymer with the monomeric amide, it was concluded that the transition of $\text{poly}\text{(N-}\text{methyl-}\text{l}\text{-alanine}\text{)}$ was caused by the isomerization of amide bonds. The complex n.m.r. spectrum was interpreted in terms of the distribution along the polymer chain of various non-planar amide links as well as planar cis and trans amide links. The difference of the mechanism of conformational transition between poly(amino acid) and poly(imino acid) is also discussed.     

A polarised μ-FTIR study on a model system for nylon 6 6: implications for the nylon Brill structure

Polarised transmission FTIR microscopy studies (μ-FTIR) have been performed on a monodisperse 3-amide oligomer. The oligomer is a model compound for nylon 6 6; it has essentially the same room temperature crystal structure, and it undergoes the same high temperature transition, the Brill transition, prior to melting. However, the oligoamide forms extended chain, rather than chain-folded, crystals, and so crystals are produced that are essentially 100% crystalline, and of μm–mm size. Consequently, this material is ideally suited for polarised μ-FTIR single crystal studies. The thermal polarised FTIR behaviour of this material provides definitive proof that the Brill transition does not involve major rearrangement of hydrogen bonds, since the strong parallel polarisation of both the NH stretch and amide I bands are retained right up to melting. Quantitative infrared dichroism measurements indicate that a maximum of 5° rotation of the N–H bonds about the extended chain axis occurs prior to melting. These results strongly suggests that the equivalent Brill transition in nylon 6 6 also proceeds without significant hydrogen bond rearrangement. In addition we have investigated the behaviour of designated ‘Brill’, ‘crystalline’, ‘amorphous’ and ‘fold’ bands that are present in our spectra.     

The structures of poly(oxyethylene)s having sulfone groups in the side chains

The structures of (OCH₂CHR) with R=CH₂S(CH₂)₆SO₂(CH₂)_MH (ATP-M; M=5,7,9) or R=CH₂SO₂(CH₂)₆SO₂(CH₂)_MH (ASP-M; M=5,7,9) were studied using X-ray diffraction and differential scanning calorimetry. The X-ray patterns of all ATPs and ASPs studied show a series of ordered reflections in the small angle region and a sharp wide angle reflection at d=∼4.4 Å, characteristic of a smectic phase. The smectic layer thickness corresponds to twice the most extended side chain length and linearly increases as the side chain length increases with a slope of ∼2.3 Å per methylene spacer. This indicates that all ASPs and ATPs studied have a double layer structure with side chains normal to the main chain and probably an all-trans conformation of the side chains. The correlation lengths measured from the wide angle reflections are in the range of 80±10 Å for all the polymers except for ASP-5 (∼40 Å). These values indicate that quasi-long-range order exists in the smectic layers whose structures can be defined as smectic B (S_B). The d-spacing of the wide angle reflection, 4.4 Å, suggests that paraffinic side chain crystallization does not occur and that the smectic mesophase develops through dipole-dipole interactions of sulfone groups in the side chains. During heating, ATP-5 shows recrystallization after the first melting. The structure produced during recrystallization has a similar smectic structure but with more dense packing between side chains than before the first melting. In the case of ASP-9, a smectic to smectic transition was observed at ∼110°C prior to the isotropic temperature at ∼150°C. Both the correlation length (from the wide angle reflection) and the layer thickness decreased from ∼80 to ∼30 Å and from 46 to 40 Å at this transition, respectively, indicating that the order in the smectic layers is lost and the S_B structure has become a less ordered S_A structure at this transition.     

Synthesis and characterization of polyethylene-like polyurethanes derived from long-chain, aliphatic α,ω-diols

Long-chain aliphatic α,ω-diols containing up to 32 consecutive methylene groups were synthesized by several methods and characterized. 1,22-Docosanediol HO-(CH₂)₂₂-OH and 1,32-dotriacontanediol HO-(CH₂)₃₂-OH both exhibited a solid-solid phase transition before melting. The α,ω-diols HO-(CH₂)_m-OH, where m=12, 22, or 32, were reacted in the melt with much shorter aliphatic α,ω-diisocyanates OCN-(CH₂)_n-NCO, where n=4, 6, 8, or 12, producing a series of linear, aliphatic, and increasingly polyethylene-like m,n-polyurethanes. Characterization (by DSC, TGA, and SAXS) of the m,n-polyurethane series showed that when the aliphatic segments were increased, and the hydrogen-bonding densities thus decreased, the polymers displayed physical and thermal properties (for example, solubility and melting temperature) typical of polyethylene.     

Structure and ferroelectric phase transition of vinylidene fluoride-trifluoroethylene copolymers: 2. VDF 55% copolymer

Molecular and crystal structure changes in ferroelectric phase transition of vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer with a VDF content of 55 mol% have been investigated by X-ray diffraction and infra-red and Raman spectroscopy. As the temperature rises from room temperature to the Curie point of ～60°C, the polar low-temperature phase consisting of all-trans chains experiences a first-order transition to the phase of tilted long trans segments connected by some skew bonds. At higher temperature this new phase transforms continuously and steeply to the non-polar high-temperature phase, where the molecular structure consists of a random combination of TG, T?, T₃G and $\text{T}{}_3\text{G}\text{?}$ rotational sequences. In the cooling process the high-temperature phase transforms to the cooled phase, which is essentially equivalent to the phase appearing intermediately in the heating process from the low-temperature to the high-temperature phase. The cooled phase gives a tilting X-ray fibre diagram and is found by X-ray analysis to contain an appreciable amount of so-called 60° domain structure. These are well interpreted by a conformational model of tilting trans structure containing skew linkages. The tensile stress along the fibre axis causes the transformation from the cooled phase to the low-temperature phase, where the probability of 60° domain structure and the degree of chain tilting are remarkably reduced.     

Substituted poly(silylenemethylene)s with short range interactions that induce a preference for the same local conformation in unperturbed atactic, isotactic, and syndiotactic chains

A rotational isomeric state model has been developed for the poly(silylenemethylenes) with repeating sequence [Si(CH₃)R-CH₂]_x, R=-O(CH₂)_NOC₆H₄C₆H₅. The model incorporates all first- and second-order interactions, as well as higher order interactions that are mandatory in some of the conformations. Chains with all possible stereochemical sequences prefer a local conformation that is a run of trans states at the C-Si bonds in the backbone. This preference arises from an attractive second-order interaction of the first methylene group in the side-chain bonded to chain atom i with the silicon atoms indexed i±2. Unperturbed chains have larger dimensions than the simpler chain in which R is merely a methyl group. The temperature coefficients of the unperturbed dimensions are large and negative. The preference of unperturbed atactic, isotactic, and syndiotactic chains for the same local conformation may contribute to the facile formation of smectic phases by the presumably atactic chain, as reported by Park et al. [Macromolecules 35 (2002) 2776].     

Conformational properties of poly(alkene sulphone)s in solution: 1. Relation between the dielectric properties in solution and the structure of the repeat unit

A survey of the available evidence shows that the central CC bonds in β-disulphones and polysulphones have a mainly trans, and sometimes an exclusively trans, conformation.Calculations of the coulombic interactions in the 27 possible conformational sequences in SO₂CH₂CH₂SO₂ show that certain sequences having gauche CC bonds are unlikely to be significantly populated. Others such as (t, g^?, g⁺) and (g^?, g^?, g⁺) involve a certain amount of steric interaction but are likely to be comparable in energy with those having a trans CC bond. One alkyl substituent or two at the same carbon atom eliminates some but not all of the accessible sequences having a gauche CC bond. Substituents at both carbon atoms cause such strong steric interactions that only trans CC bonds are allowed. It is shown how such considerations account satisfactorily for the two types of dielectric behaviour of poly(alkene sulphone)s in solution.Preliminary experiments indicate that poly(norbornene sulphone), unlike polysulphones of other 1,2-disubstituted alkenes, does have a permanent dipole because of the inability of adjacent CS bonds to take up anti-parallel positions.A close resemblance is noted between the conformation about the main-chain CC bond in poly(cyclohexene sulphone) and that about the main-chain CC bond in poly(but-2-ene sulphone) in its most stable configuration.     

Quantification of silane grafting efficacy,weak IR vibration bands and percentage crystallinity in post e-beam irradiated UHMWPE

Vinyl-tri-methoxy silane (VTMS) and vinyl-tri-ethoxy silane (VTES) were grafted onto ultra-high molecular weight polyethylene (UHMWPE) by irradiating the UHMWPE/silane hybrids with e-beam. The samples were irradiated under high moisture contents for total dose values of 30, 65 and 100 kGy, respectively. The synergistic effect of silane and irradiation on the grafting efficacy, concentration of weak bonds like trans-vinylene (–CH=CH–) and vinyl (–CH=CH₂) and percentage values of crystallinity were studied using FTIR spectroscopy. For the estimation of grafting reactions efficiency, absorption due to characteristic infrared absorption bands of –Si–CH– in the region ~800 cm^?1 was monitored and found that grafting efficacy of VTMS on UHMWPE was higher as compared to VTES and increased with irradiation. The relative amounts of grafting extension (R) for 100 kGy irradiated UHMWPE/VTMS and UHMWPE/VTES hybrids were found to increase 20 and 15 %, respectively. The concentration of trans-vinylene in UHMWPE was found to increase from 0.015 to 0.035 mmol/l due to synergistic effects of silane and irradiation. Moreover, crystallinity of UHWMPE was found to decrease from 65 to 55 % due to the abovementioned synergistic effects which was also confirmed with DSC tests. Furthermore, oxidation index values were measured to confirm the efficacy of silane as free radical quencher via silane grafting extension reactions.     

An infra-red study of model compounds of poly(tetramethylene terephthalate)

An infra-red spectroscopic investigation of the poly(tetramethylene terephthalate) model compounds butyl-dibenzoate: C₆H₅COO(CH₂)₄OCOC₆H₅ (i), 1,4-bis-butyl terephthalate: HO(CH₂)₄-OCO₆H₄COO(CH₂)₄-OH (ii), α-butyl-ω-butoxyterephthalaoyl-di-(tetramethylene terephthalate): H(CH₂)₄(OCOC₆H₄COO-(CH₂)₄-)₃-OH (iii), and α-butyl-ω-butoxyterephthalaoyl-tetra-(tetra-methylene terephthalate): HO(CH₂)₄-(OCOC₆H₄COO(CH₂)₄-)₅-OH (iv) is presented. The synthesis of (i) is also described. Spectroscopic evidence indicates that the crystal conformations of the aliphatic segments in (i) and (ii) closely resemble the extended all trans beta form of PTMT. Oligomers (iii) and (iv), however, are found to be more similar to the crumpled alpha crystal phase of PTMT.     

Critical role of the conformation of comonomer units in isomorphic crystallization of poly(hexamethylene adipate-co-butylene adipate) forming Poly(hexamethylene adipate) type crystal

The isothermal crystallization of isomorphic Poly(hexamethylene adipate-co-butylene adipate) [P(HA-co-BA)], with the HA unit content ranged from 100 to 45 mol%, forming the Poly(hexamethylene adipate) [PHA] type crystal was investigated with DSC, FTIR and WAXD. The BA units were found adopting their all-trans conformation in the crystalline phase of PHA type crystal. The inclusion of the BA units into the PHA type crystal was highly preferred in the isothermal crystallization at 25 °C. The exclusion of the BA units from the crystalline phase of the PHA type crystal was enhanced by elevating the crystallization temperature (T_c), depending on the content of comonomer units. Increasing the HA unit content enhances the formation of the all-trans conformation of the BA units in the PHA type crystal. At low T_c, such an enhancement significantly helps in retarding the exclusion of the BA units from the PHA type crystalline lattice. On the other hand, at high T_c, the difficulty for forming the stable all-trans conformation of the BA units in cocrystal increased and hence the exclusion of the BA units from the cocrystal was accelerated. In conclusion, the all-trans conformation of the BA units can also play a critical role in the isomorphic crystallization of the P(HA-co-BA)s forming the PHA type crystal.     

Polarography of conjugated unsaturated lipids

Conjugated fat-soluble vitamins, methylenic interrupted and conjugated fatty acids were polarographically investigated in both basic and neutral solvents. The half-wave potentials of all-trans-retinol, 13-cis-retinol, all-trans-retinyl acetate, all-trans-retinal, and Vitamin D₂ and D₃ were related to the number of double bonds in conjugation and their geometrical configuration. A minimum of three double bonds in conjugation and their geometrical configuration. A minimum of three double bonds in conjugation was required before reduction took place at the cathode, and as the number of conjugated bonds increased in the lipid compounds, the initial reduction wave took place at a lower half-wave potential. Investigation of conjugated double bonds in triglycerides and in alkali-isomerized linolenic and arachidonic acids gave reduction waves the half-wave potentials of which were related to the number of double bonds in conjugation. In both basic and neutral solvents there was a minimum of three double bonds in conjugation necessary to obtain a reduction wave at the dropping mercury electrode. Ultraviolet absorption curves of the prolonged reduction of polyunsaturated conjugated fatty acids indicate a step-wise reduction of each end of the polyunsaturated conjugated double bonds. In neutral solvent the log of the conjugated double bonds versus the half-wave potential (versus mercury pool anode) gave a linear equation, E₁=2.98−1.6 log C. A proposed mechanism for the step-wise reduction of conjugated lipids is presented and discussed. Presented in part at AOCS Meeting, Chicago, October 1964.     

IR-spectroscopic investigations of cycloalkanes

Summary A homologous series of cycloalkanes from (CH₂)₁₄ up to (CH₂)₉₆ as model systems for tight folding in CH₂-chain molecules was investigated by infrared spectroscopy. In comparison with the corresponding spectra of the nalkanes we found two additional new absorption bands at 700 cm^–1 and 1442 cm^–1 which can be assigned to a characteristic fold vibration. The already known fold band in the wagging region at about 1344 cm^–1 could also be found in all cycloalkane spectra. A numerical decomposition of the bands enables the calibration of the fold concentration. The Davydov splitting of the (CH₂)-rocking and (CH₂)-bending vibration in the melt crystallized form of the molecules (CH₂)₇₂ and (CH₂)₉₆ show the occurence of a orthorhombic subcell of the larger rings whereas the smaller rings and the larger ones in the solution crystallized form only crystallize in a monoclinic form.presented at the Frühjahrstagung der DPG und ÖPG, march 24, 1980 in Leoben, Österreich