Segmental dynamics of interfacial poly(methyl acrylate)-d(3) in composites by deuterium NMR spectroscopy.

The interface in composite materials containing an ultrathin layer of poly(methyl acrylate)-d(3) (PMA-d(3)) on silica was studied using deuterium NMR. PMA-d(3) was deposited from solution at saturation coverage from toluene onto silica. The samples were dried and composite samples made by hot pressing the PMA-d(3)/silica samples with hydrogenated polystyrene (PS) and high (HMW) and low (LMW) molecular weight hydrogenated poly(methyl acrylate) (PMA) as the overlayer. The interfacial layers of PMA-d(3) were studied at the air-polymer-silica and polymer-polymer-silica interfaces using deuterium solid-state quadrupole-echo NMR and the results compared to those for the bulk polymer. It was found that for samples at the air-polymer-silica interface, some of the polymer segments in the surface sample had segmental mobility higher than that of the corresponding bulk PMA-d(3) sample at the same temperature. When overcoated with unlabeled polymer, the interfacial polymer at the polymer-polymer-silica interface showed reduced mobility due to the presence of the overlayer. The adsorbed PMA-d(3), in the composite samples, decreased in mobility in the order of LMW-PMA > HMW-PMA > PS. The PS sample caused the greatest reduction in the PMA-d(3) interfacial mobility. The order was consistent with the segmental mobilities of the polymers used for the overlayers. The lower the mobility of the polymer used for the overlayer, the more restricted were the polymer segments in the adsorbed PMA-d(3) layer.     

Segmental dynamics in poly(methyl acrylate) on silica: molecular-mass effects

The effect of molecular mass on the segmental dynamics of poly(methyl acrylate) (PMA) adsorbed on silica was studied using deuterium quadrupole-echo nuclear magnetic resonance (NMR) and modulated differential scanning calorimetry. Samples adsorbed on silica (all about 1.5 mg PMA/m2 silica) were shown to have more restricted segmental mobility, and higher Tg's, than the corresponding bulk PMA samples. Around the glass-transition region, adsorbed samples exhibited segmental mobility, which could be classified as heterogeneous due to a superposition of more-mobile and less-mobile components present in the deuterium NMR spectra. This heterogeneity was consistent with a motional gradient with more-mobile segments near the polymer-air interface and the less-mobile species near the polymer-silica interface. The mobility of the adsorbed 77 kDa PMA sample was the lowest among the four different molecular-mass samples studied. Samples studied with masses both larger and smaller than 77 kDa had larger mobile-component fractions in the adsorbed polymer. The additional mobility was attributed to the presence of either longer tail and loop conformations in the higher molecular-mass samples or the inherent mobility of the tails in the lower molecular-mass samples on the surface.     

Dynamics of adsorbed poly(methyl acrylate) and poly(methyl methacrylate) on silica

Deuterium NMR and modulated differential scanning calorimetry (MDSC) were used to probe the behavior of ultrathin adsorbed poly(methyl acrylate) (PMA). The spectra for the bulk methyl-labeled PMA-d₃ were consistent with the motions of the polymer segments being spatially homogeneous. For the polymers adsorbed on silica, multicomponent line shapes were observed. The segmental mobility of the surface polymers increased with increased adsorbed amounts. In contrast to the behavior of the polymers in bulk, the adsorbed lower-molecular-mass PMA-d₃ was less mobile than the adsorbed high-molecular-mass polymer. The presence of a polymer overlayer was sufficient to suppress the enhanced mobility of the more-mobile segments of the adsorbed (inner) polymer. MDSC studies on adsorbed poly(methyl methacrylate) showed that the glass-transition temperature of the thin polymer films increased and broadened compared to the behavior of the polymer in bulk. The presence of a motional gradient with the less-mobile segments near the solid-polymer interface and the more-mobile segments near the polymer-air interface was consistent with the experimental observations.     

Microstructural study of poly(methyl methacrylate-co-n-propyl acrylate) by 13C NMR

Analysis of carbonyl and β-CH₂ signals in the 100?MHz ¹³C NMR spectra of poly(methyl methacrylate-co-n-propyl acrylate) (PMMA/nPrA), provided distribution of configurational-compositional sequences for a series of the copolymer samples of different composition at pentad level for carbonyl signal and hexad level for the backbone methylene carbons. Computer simulation of the spectra based on incremental calculation of the chemical shifts for individual sequences provided very good agreement with the experimental spectra.     

Stereoregularity of poly(methyl acrylate)

The stereoregularity of poly(methyl acrylate) and poly(methyl acrylate-αd) was determined from the NMR spectra. A method of quantitative determination of stereoregularity of poly(methyl acrylate) proposed in this paper is based on the fact that in the 100 Mc./sec. NMR spectrum the absorption peaks due to methylene protons in syndiotactic configurations overlap absorptions due to only one of two methylene protons in isotactic configurations. The stereostructure of poly(methy1 acrylates) polymerized with anionic catalysts such as Grignard reagents, n-butyllithium, and LiAlH₄ is generally richer in isotactic diads than in syndiotactic diads. For example, poly(methyl acrylate) polymerized with phenylmagnesium bromide as catalyst at ?20°C. consists of 99% isotactic and 1% syndiotactic diads. In radical polymerization, the isotacticity of poly(methyl acrylate) is independent of polymerization temperature. Poly(methyl acrylates) polymerized with a Ziegler-Natta catalyst consisting of Al(C₂H₅)₂Cl and VCl₄ have configurations similar to those polymerized by radical initiators. The stereoregularity of poly(methyl acrylate-α-d) resembled that of poly(methyl acrylate) polymerized under the same conditions.     

Segmental motions of poly(ethylene glycol) chains adsorbed on Laponite platelets in clay-based hydrogels: a NMR investigation

The segmental dynamics of poly(ethylene glycol) (PEG) chains adsorbed on the clay platelets within nanocomposite PEG/Laponite hydrogels was investigated over the tens of microseconds time scale, using combined solution and solid-state NMR approaches. In a first step, the time evolution of the molecular mobility displayed by the PEG chains following the addition to a Laponite aqueous dispersion was monitored during the aggregation of the clay disks and the hydrogel formation, by means of (1)H solution-state NMR. Part of the PEG repeat units were found to get strongly constrained during the gelation process. Comparisons between this time evolution of the PEG local dynamics in the PEG/Laponite/water systems and the increase of the macroscopic storage shear modulus, mainly governed by the assembling of the Laponite disks, indicate that the slowing down of the segmental motions arises from adsorbed PEG repeat units or chain portions strongly constrained between aggregated clay layers. In a second step, after completion of the gelation process, the molecular motions of the adsorbed PEG chains were probed by (1)H solid-state NMR spectroscopy. (1)H double-quantum experiments indicate that the adsorbed PEG repeat units, though reported to be frozen over a few tens of nanoseconds, still display significant reorientational motions over the tens of microseconds time scale. Using a comparison with a model system of amorphized PEG chains, the characteristic frequency of these segmental motions was found to range between 78.0 kHz and 100.7 MHz at 300 K. Interestingly, at this temperature, the level of reorientational motions detected for these adsorbed PEG chain portions was found to be as restricted as the one of bulk amorphous PEG chains, cooled at a slightly lower temperature (about 290 K).     

Structural investigation of poly (methyl acrylate) by 2D HMBC NMR

Three and four bond order couplings between carbonyl carbon and other neighboring protons have been investigated in detail with the help of 2D heteronuclear multiple bond correlation (HMBC) spectrum. The methine and methylene protons in pentad and tetrad configuration were found to couple with carbonyl carbon. The intensity ratio of methylene protons’ cross peaks in HMBC spectrum shows that 50% of poly methyl acrylate (PMA) is in isotactic and 50% is in syndiotactic configuration. The configurational analysis was mainly carried out with the help of 2D HMBC spectrum.     

Microstructure of poly(methyl methacrylate-co-isopropyl acrylate) studied by 13C NMR

The distribution of configurational–compositional sequences of poly(methyl methacrylate-co-isopropyl acrylate) (PMMA/iPrA) has been determined from the carbonyl and β-CH₂ signals in the 100?MHz ¹³C NMR spectra of the copolymer. The carbonyl signal provided information on configurational–compositional sequences up to heptads, whereas β-CH₂ signals offered complementary information on even sequences up to hexads. The assignment of the sequences to the respective signals was based on a comparison with the spectra of respective homopolymers, that is, PMMA and PiPrA followed by a computer simulation applying an incremental calculation of chemical shifts of the individual sequences.     

Solution properties of poly(methyl acrylate) and (1:1) poly(styrene-co-methyl acrylate)

Poly(methyl acrylate) (PMA) and 1:1 poly(styrene-co-methyl acrylate) (PSMA) were prepared by solution and bulk polymerization, respectively. The copolymer was analyzed with NMR to ascertain its composition and microstructure. The solution properties of unfractionated PMA and fractionated PSMA in ethyl acetate were investigated by light-scattering and viscosity techniques at 35°C. Narrow composition heterogeneity as revealed from the light-scattering measurements in different solvents justified the use of a single solvent for the copolymer characterization. The equations relating the limiting viscosity number to molecular weight, the molecular dimension to molecular weight, etc., were found for homopolymer and copolymers in ethyl acetate at 35°C. In the evaluation of the Flory constant K for the unperturbed state by methods based on Flory-Fox-Schaefgen, Kurata-Stockmayer, and Stockmayer-Fixman expressions, only the first method gave a value for PMA in ethyl acetate, consistent with that obtained in other solvents, whereas similar values of K were obtained by the three methods for PSMA in ethyl acetate. The studies indicate reduced thermodynamic interaction for PSMA–ethyl acetate compared to PMA–ethyl acetate, but increased steric effect in the copolymer compared with the homopolymer.     

NMR measurement of the surface dynamics of poly(dimethylsiloxane) adsorbed on silica gel

CPMAS-DD ¹³C NMR spectroscopy was used to examine the mobility of poly(dimethylsiloxane) adsorbed on silica gel (PDMS/SiO₂) at submonolayer coverages. The spin-lattice relaxation time in the rotating frame (T_1ρH) decreased linearly with increasing loading. This is consistent with a decrease in the mobility of the polymer segments as the loading is increased. The decrease in mobility results from interpolymer interference. We propose a model that explains these results in terms of a surface intrinsic viscosity that incorporates the polymer-polymer interactions on the surface.     

Molecular mass and dynamics of poly(methyl acrylate) in the glass-transition region

The segmental dynamics of bulk poly(methyl acrylate) (PMA) were studied as a function of molecular mass in the glass-transition region using 2H NMR and modulated differential scanning calorimetry (MDSC). Quadrupole-echo 2H NMR spectra were obtained for four samples of methyl-deuterated PMA-d3 with different molecular masses. The resulting spectra were fit using superpositions of simulated spectra generated from the MXQET simulation program, based on a model incorporating nearest-neighbor jumps from positions on the vertices of a truncated icosahedron (soccer-ball shape). The lower molecular-mass samples, influenced by the presence of more chain ends, showed more heterogeneity (broader distribution) and lower glass transitions than the higher molecular-mass samples. The MDSC experiments on both protonated and deuterated samples showed behavior consistent with the NMR results, but temperature shifted due to the different frequency range of the measurements in terms of both the position and breadth of the glass transition as a function of molecular mass.     

Thermosensitive properties of a novel poly(methyl 2-acetamidoacrylate-co-methyl acrylate)

Copolymerizations of methyl 2-acetamidoacrylate (MAA) with methyl acrylate (MA) were carried out at 60 °C in chloroform. MAA-rich copolymers are soluble in water and MAA-poor copolymers insoluble. Among water-soluble copolymers obtained, only one (HP-77) which contains 77% of MAA units was thermosensitive. Thermal properties of HP-77 were investigated in the presence or absence of inorganic salts. The cloud point of aqueous HP-77 solution depended on polymer concentration: The cloud point decreased exponentially with an increasing concentration of the polymer. The cloud point of HP-77 was also affected significantly by the type and concentration of salts. The effectiveness of salts to reduce the cloud point is NaBr≈KBr<NaCl≈KCl<Na₂SO₄≈K₂SO₄. The salting-out coefficients were evaluated as 2.45 l/mol for sodium chloride and 14.56 l/mol for sodium sulfate, respectively, from the relationship (Setschenow's equation) between logarithm of the solubility of HP-77 and salt concentration. The salting-out coefficient of sodium sulfate is larger than that of sodium chloride.     

Surface segregation of poly(2-methoxyethyl acrylate) in a mixture with poly(methyl methacrylate)

Poly(2-methoxyethyl acrylate) (PMEA) exhibits excellent blood compatibility. To understand why such a surface functionality exists, the surface of PMEA should be characterized in detail, structurally and dynamically, under not only ambient conditions, but also in water. However, a thin film of PMEA supported on a solid substrate can be easily broken, namely it is dewetted. Our strategy to overcome this difficulty is to mix PMEA with poly(methyl methacrylate) (PMMA). Differential scanning calorimetry and cloud point measurements revealed that the PMEA/PMMA blend has a phase diagram with a lower critical solution temperature. The blend surface was also characterized by X-ray photoelectron spectroscopy in conjunction with microscopic observations. Although PMEA is preferentially segregated over PMMA at the blend surface due to its lower surface free energy, the extent of segregation in the as-prepared films was not sufficient to cover the surface. Annealing the blend film at an appropriate temperature, higher than the glass transition temperature and lower than the phase-separation temperature of the blend, enabled us to prepare a stable and flat surface that was perfectly covered with PMEA.     

Atmospheric ozonolysis study of methyl acrylate and methyl 3-methyl acrylate

Detailed reaction mechanisms of methyl acrylate (MA) and methyl 3-methyl acrylate (M3MA) with ozone have been investigated using quantum chemistry calculations based on the CCSD(T)/6-31G(d)+CF//B3LYP/6-31+G(d,p) level of theory. Possible reaction channels and products have been presented and discussed. The temperature-dependent and pressure-independent overall and site-specific rate constants are calculated by employing multichannel RRKM theory. The obtained overall rate constants (in cm³ molecule^?1 s^?1) based on the CCSD(T)+CF energies can be described as: $ k_{{({\text{MA}} + {\text{O}}_{ 3} )}} = 3. 7 4 \times 10^{ - 1 3} { \exp }\,( - 3 7 4 6. 1 2/T) $ and $ k_{{({\text{M3MA}} + {\text{O}}_{ 3} )}} = 5. 1 2 \times 10^{ - 1 3} { \exp }( - 3 3 5 4. 8 7/T) $ at 200–400 K and 760 Torr. Under atmospheric conditions, the dominant products of ozonolysis of MA and M3MA are methyl glyoxylate and formaldehyde, methyl glyoxylate and acetaldehyde, respectively. The results of theoretical study are in good agreement with the available experimental measurements. Researches on branching ratios and atmospheric lifetimes also have been obtained as complement to the experimental results.     

Self-stratified films obtained from poly(methyl methacrylate/n-butyl acrylate) colloidal dispersions containing poly(vinyl alcohol): a spectroscopic study

These studies focus on the role of poly(vinyl alcohol) (pVOH) during colloidal synthesis of poly(methyl methacrylate/n-butyl acrylate) (pMMA/nBA) and its effect on particle coalescence. Using 2D photoacoustic FT-IR spectroscopy and internal reflection IR imaging, we showed that the presence of pVOH creates competing environments between the copolymer particle surfaces, aqueous phases, and dispersing agents which results in migration and self-induced stratification occurring during coalescence. pMMA/nBA/pVOH films stratify to form sodium dodecyl sulfate rich film-air interfaces, and the -SO3- moieties exhibit preferential parallel orientation with respect to the surface. At the same time, the bulk of the film is dominated by intramolecular hydrogen bonding between the pVOH phase and the copolymer matrix. This behavior is attributed to significant interactions between pVOH and pMMA/nBA, resulting in limited mobility of pVOH.     

Calorimetric study of free-radical polymerized poly(p-biphenyl acrylate) and poly(p-cyclohexylphenyl acrylate)

DSC traces and specific heat data for poly(p-diphenyl acrylate) (PPBA) and poly(p-cyclohexylphenyl acrylate) (PPCPA) obtained by radical polymerization are reported. The results indicate the existence of a definite ordered phase and of a reversible firstorder solid–liquid transition in both polymers although x-ray diffraction studies showed that they are not crystalline in the conventional sense. The extent of the ordered phase present in each polymer is calculated, and the problems involved in such determination by thermal measurements are discussed. On the basis of the experimental results reported here in conjunction with the x-ray data, models are proposed for the morphology of these polymers.     

NMR study of poly(vinyl chloride)-β,β-d2

The high-resolution NMR spectra at 60 and 100 Mcps of poly(vinyl chloride)-β,β-d₂ in o-dichlorobenzene, pyridine, and C₂HCl₅ solutions are reported. The use of low molecular weight samples and of {D} spin-decoupling experiments, which yield higher resolution spectra, results in the observation of a number of additional resonances for the α-proton. These can be interpreted in terms of pentad configurational sequences of monomer units. It is found that, whereas the S syndiotactic pentads cannot be resolved, two components of the H heterotactic and all of the possible I isotactic pentads are clearly discernible. From the tacticity values of polymers prepared at +40, 0, and ?40°C, enthalpy and entropy of activation for isotactic and syndiotactic monomer placement are found to be 630 cal/mole and 1.5 eu, respectively.     

Photoiniferter surface grafting of poly(methyl acrylate) using xanthates

The UV‐induced and catalyst‐free photoiniferter surface grafting using xanthate RAFT agents is studied. First, a novel silane containing a xanthate RAFT agent moiety is synthesized and successfully grafted onto a silicon wafer as confirmed by secondary ion mass spectrometry (SIMS). Next, using only methyl acrylate, solvent and UV light (365 nm) polymer brushes are grafted rapidly from the surface with film thicknesses up to 25 nm reached within half an hour. The obtained polymer films are thoroughly analyzed by X‐ray photoelectron spectroscopy (XPS), ToF‐SIMS, and AFM. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 2002–2007