Effect of propyleneimine external cross-linker on the properties of acrylate latex pressure sensitive adhesives

Acrylate pressure sensitive adhesive (PSA) latexes were synthesized via a starved monomer seeded semi-batch emulsion polymerization process with butyl acrylate (BA), methyl methacrylate (MMA), acrylic acid (AA) and 2-hydroxyethyl acrylate (HEA). These PSA polymers were then cross-linked with trifunctional propyleneimine external cross-linker (SAC-100) to study the cross-linking reaction between carboxylic group of the polymer chain and cross-linking agent. It was found that cross-linking provided a significant influence on the film formation process based on the result of SEM analysis. In addition, with the increase of SAC-100 content, the gel content of the polymer increased significantly, while molecular weight between cross-link points (M_c) and the sol molecular weight (M_w, M_n) of the polymer decreased remarkably. The TGA result showed that the addition of the external cross-linker can enhance the thermal stability of the latex film. Moreover, for the cross-linked adhesive film, the shear strength was improved greatly while at the sacrifice of loop tack and peel strength, when compared with the uncross-linked counterparts. Besides, dynamic mechanical analysis (DMA) was also used to evaluate the viscoelastic properties of the acrylate emulsion PSA film.     

Pulsed electron beam irradiation of dilute aqueous poly(vinyl methyl ether) solutions

A dilute aqueous solution of the temperature-sensitive polymer, poly(vinyl methyl ether) (PVME), was irradiated by a pulsed electron beam in a closed-loop system. At temperatures, below the lower critical solution temperature (LCST), intramolecular crosslinked macromolecules, nanogels, were formed. With increasing radiation dose D the molecular weights M_w increase, whereas the dimensions (radius of gyration R_g, hydrodynamic radius R_h) of the formed nanogels decrease. The structure of the PVME nanogels was analyzed by field emission scanning electron microscopy (FESEM) and globular structures with d=(10-30) nm were observed. The phase-transition temperature of the nanogels, as determined by cloud point measurements, decreases from T_cr=36 °C (non-irradiated polymer) to T_cr=29 °C (c_p=12.5 mM, D=15 kGy), because of the formation of additional crosslinks and an increase in molecular weights. The same behavior was observed for a pre-irradiated PVME (γ-irradiation) with higher molecular weight due to intermolecular crosslinks. After pulsed electron beam irradiation the molecular weight again slightly increases whereas the dimension decreases. Above D=1 kGy the calculated ρ-parameter (ρ=R_g/R_h) is in the range of ρ=0.5-0.6 that corresponds to freely draining globular structures.     

Copoly(m,p-chloromethylstyrene-50% styrene): Copolymer molecular weights,fractionation and derivatization

Copolymerization of an equimolar mixture of m,p-chloromethylstyrene ($\text{M}$₁) and styrene ($\text{M}$₂) was carried out in chlorobenzene in the presence of AIBN at 80°C. Molecular weight analysis (by g.p.c.) of the resulting polymer samples was performed at various conversions. $\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}$, $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$, and ($\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$) value of 21 300, 13 800 and 1.54 were obtained at 8.9% conversion. At higher conversions, the value of $\text{M}\text{?}{}_{\mathrm{<mtext>w</mtext>}}$ remained effectively constant while $\text{M}\text{?}{}_{\mathrm{<mtext>n</mtext>}}$ decreased to 9200 at ca. 80% conversion, and then increased to 12 000 at about 100% conversion (16 h), and to 13 700 if the polymer solutions were maintained at 80°C for an additional 44 h. These results suggest that, although the termination step initially involves the combination of polymer radicals, at high conversions a large number of very low molecular weight, and unsaturated, polymer molecules are formed possibly by disproportionation involving polymer radicals and primary radicals. The unsaturated polymer molecules are subsequently polymerized by growing polymer radicals towards the end of the polymerization. It was noticed that further reaction occurred after complete depletion of monomer, involving radical attack on the unsaturated polymer molecules. Other reactions including chain transfer to polymer will also be important at high polymer concentrations. A copolymer of $\text{M}$₁ and $\text{M}$₂ was separated into four fractions on a preparative scale, and molecular weight analysis of the resulting polymer samples provided more evidence of the above interpretation. G.p.c. analysis of several derivatives of a copolymer of $\text{M}$₁ and $\text{M}$₂ showed that most molecular weights were much lower than that of the starting polymer. These results in some cases may reflect the chemical or dimensional changes introduced into the polymer molecules during derivatization.     

Control of main chain structure and possibility of molecular weight control of polymer on polymerization of vinyl chloride with tert-butyllithium

The controlled polymerization of vinyl chloride (VC) with tert-butyllithium (tert-BuLi) was investigated. The polymerization of VC with tert-BuLi at −30 °C proceeded to give a high molecular weight polymer in good yield. In the polymerization of VC −30 to 0 °C under nearly bulk, the relationship between the M_n of polymers and polymer yields gave a straight line passed through the origin, but the M_w/M_n of PVC was not narrow. When CH₂Cl₂ was used as polymerization solvent, the M_n of PVC increased with the polymer yield, and the M_w/M_n of 1.25 was obtained. Structure analysis of the resulting polymers indicates that the main chain structure could be regulated in the polymerization of VC with tert-BuLi. Accordingly, a control of molecular weight of polymer and main chain structure is possible in the polymerization of VC with tert-BuLi.     

Small-angle neutron scattering from symmetric blends of poly(dimethylsiloxane) and poly(ethylmethylsiloxane)

We present results of a small-angle neutron scattering (SANS) study of the structure and thermodynamic properties of symmetric blends of deuterated poly(dimethylsiloxane) (d-PDMS) and poly(ethylmethylsiloxane) (PEMS) as a function of temperature (T) (40≤T≤300 °C) and the molecular weight (M_w) (4700≤M_w≤23,200). The radius of gyration (R_g) of d-PDMS was measured using the high-concentration labeling method and revealed unperturbed chain dimensions at all temperatures regardless of the polymer M_w. The random phase approximation (RPA) fits the data for low M_w blends, however it fails to describe the SANS data for M_w>10,000 g/mol. This observation is explained by the fact that for high M_w blends the correlation length of the concentration fluctuations ξ is always large (ξ>R_g), implying that these blends remain microscopically inhomogeneous at all temperatures studied in this work. At the same time, the low M_w blends are randomly mixed (ξ<R_g) at all T and can reach the ‘ideal mixing’ or Θ condition (χ=0).     

Molecular weight influence on shape memory effect of shape memory polymer blend (poly(caprolactone)/styrene-butadiene-styrene)

The shape memory effect (SME) does not only concern the macroscopic structure. It concerns also the polymer structure at morphological, macromolecular, and molecular scales. This effect may depend on different physicochemical properties like morphology heterogeneity, chain rigidity, steric hindrance, chain polarity, free volume, cross-linking or entanglement density, molecular shape and weight, and so on. Hence, finding the relationship between the SME and these properties is very important. This can help to obtain the knowledge about the phenomenon origin and mechanism. One of the basic polymer properties, which can have direct SME, may be the molecular weight (M_w ). The question here is: If the M_w of a shape memory polymer (SMP) changes, for different reasons like degradation, what will be the effect of this change on its SME. In order to answer to this question, the investigation is focused on an SMP blend of 40% poly(ɛ-caprolactone) (PCL) and 60% styrene-butadiene-styrene (SBS). Then, enzymatic hydrolysis is performed on this blend to change its M_w . It is shown that this change is only related to the variation in the M_w of PCL. After that, different samples with a distinct average M_w are prepared and characterized by various experimental methods. Shape memory tests are performed on these blends, and the recovery rate (R_r ) for each of them is determined. It is found that when M_w of PCL decreases, its degree of crystallinity, its glass transition, and its melting temperatures, corresponding to the PCL phase, increase. However, the elongation at break of the blend declines with the reduction in M_w . The tests show that the alteration in the blend's M_w influences its SME. Indeed, R_r of the (PCL/SBS) mixture drops with the decrease in M_w of PCL.     

Polystyrene-equivalent molecular weight versus true molecular weight in size-exclusion chromatography

The evaluation of the size-exclusion chromatography (SEC) concentration elution curves by means of a calibration dependence obtained in a given SEC set for a polymer different from the polymer to be analyzed results in an error in the determination of both molecular weight and molecular-weight distribution (MWD). The problem is analyzed assuming the validity of the universal-calibration concept. The differences between the true and apparent values of molecular weight, MWD and M_w/M_n depend on and are expressed in terms of the parameters of the Mark-Houwink-Kuhn-Sakurada equation, describing the molecular-weight dependence of intrinsic viscosity, for the polymer to be analyzed and the polymer used for calibration. The differences in molecular weight and the M_w/M_n ratio are typically tens of percent and, in extreme cases, can amount up to a factor of three for molecular weight and a factor of two for the M_w/M_n ratio.     

Role of chain entanglements on fiber formation during electrospinning of polymer solutions: good solvent, non-specific polymer-polymer interaction limit

Chain entanglements are one of many parameters that can significantly influence fiber formation during polymer electrospinning. While the importance of chain entanglements has been acknowledged, there is no clear understanding of how many entanglements are required to affect/stabilize fiber formation. In this paper, polymer solution rheology arguments have been extrapolated to formulate a semi-empirical analysis to explain the transition from electrospraying to electrospinning in the good solvent, non-specific polymer-polymer interaction limit. Utilizing entanglement and weight average molecular weights (M_e, M_w), the requisite polymer concentration for fiber formation may be determined a priori, eliminating the laborious trial-and-error methodology typically employed to produce electrospun fibers. Incipient, incomplete fiber formation is correctly predicted for a variety of polymer/solvent systems at one entanglement per chain. Complete, stable fiber formation occurs at ≥2.5 entanglements per chain.     

Diffusion of probe polystyrenes with different molecular weights in poly(methyl methacrylate) gels and inhomogeneity of the network structure as studied by time-dependent diffusion NMR spectroscopy

Poly(methyl methacrylate) (PMMA) gels have been prepared with radical polymerization by cross-linking methyl methacrylate monomer using ethylene glycol dimethacrylate as cross-linking monomer in toluene containing polystyrenes (PSs) with M_w from 4000 to 400,000. The diffusion coefficients of the PSs in the PMMA gels swollen in deuterated chloroform have been measured by pulsed field-gradient (PFG) ¹H NMR method with the diffusing time Δ varied. From the experimental results, it is found that the network structure of PMMA gels prepared in the presence of PSs with M_w=4000 and 400,000 are relatively homogeneous and inhomogeneous, respectively, within the Δ range from 40 to 500 ms.     

Change in the molecular weight distribution of poly(ethylene oxide) caused by the complexation with poly(acrylic acid)

The complexation between poly(ethylene oxide) (PEO) and poly(acrylic acid) (PAA) was made by using double the molar quantity of either polymer component at pH 2 where the resulting complex completely precipitates. After the removal of the precipitate, PEO or PAA remaining in the supernatant was subjected to gel permeation chromatography to investigate the change in the molecular weight distribution (MWD) caused by the complexation. No remarkable difference is observed in the MWD curves for PEO[1] (M_w=1.37 × 10⁴) before and after the complexation with PAA[1] (M_w=1.10 × 10³) and PAA[2] (M_w=4.16 × 10⁵). However, the MWD curves of PEO[2] (M_w=1.26 × 10⁵) and PAA[2] become shortened and shift to the low molecular weight side after the complexation with PAA[1] or [2] and PEO[2], respectively. This tendency is enhanced by increasing the complexation temperature. From these results, it is indicated that the complexation between PEO and PAA deals with an equilibrium reaction, and the equilibrium constant is dependent on the chain length of both polymer components and also on the complexation temperature.     

Fluorescence response from anthracene labeled polystyrene to study its thermal transitions

In this work, thermal transitions of polystyrene, PS, were studied as a function of molecular weight (M_w) and thermal history using the fluorescence response of anthracene chemically bonded to the polymer chain ends. For each PS sample the fluorescence spectra of the anthracene were collected as a function of temperature in cycles of heating and cooling. Two photophysical parameters were selected in order to study thermal transitions of the PS samples: (i) the first moment of the fluorescence band, 〈ν〉; and (ii) the integrated fluorescence intensity, I. In every case, a sharp change in the photophysical parameter was observed at a certain temperature that was assigned to the α relaxation temperature and which was clearly different depending on the PS molecular weight and the thermal history. In fact, the relaxation temperature increased with M_w during a heating scan, decreased with M_w during a cooling scan and was clearly lower in the cooling scans. These results were interpreted in terms of a reorientation of the anthryl groups to the surface of the polymer.     

Impact of electron beam treatment on copolymers of polylactide and poly(trimethylene carbonate) in an air atmosphere

The effect of exposure of polylactide (PLA) and poly(trimethylene carbonate) (PTMC) statistical copolymers to ionizing radiation was studied by means of EPR spectroscopy. In addition, the influence of radiation-induced processes on thermal properties, miscibility of the components, weight average molecular weight (M_w) and number average molecular weight (M_n) were investigated for doses in the range of 0–200 kGy. In copolymers containing PLA and PTMC components in a ratio of 30:70 and 70:30 PLA radicals identified in the homopolymer under cryogenic conditions were dominant. This showed that PTMC radical centers either recombine or are transferred to PLA along the macromolecules. The results obtained for the non-irradiated and irradiated samples showed that the glass transition values measured by differential scanning calorimetry and calculated using the Fox equation were similar and indicated compatibility between the constituents of the tested copolymers and their miscibility. M_w and M_n changes measured by gel permeation chromatography were used to determine the radiation yield of scission G(S) and cross-linking G(X). In the case of PLA and PLA-rich copolymers, the difference between G(S) and G(X) with increasing dose increased, thus the chain scission predominated over cross-linking. For PTMC rich copolymer, the effect was opposite.     

A rheology theory and method on polydispersity and polymer long-chain branching

Model calculations were performed to investigate the sensitivity of zero-shear melt viscosity (η₀ or Eta0) on the molecular weight (MW) polydispersity of linear polymers. Simulated MW distributions (MWD) were generated with the generalized exponential (GEX) distribution function for various levels of polydispersity M_w/M_n and M_z/M_w. For linear entangled polymeric chains in the melt, the linear viscoelastic properties were predicted by using the double reptation blending rule and the so-called BSW relaxation time spectrum, named after the authors: Baumgaertel, Schausberger and Winter [Baumgaertel M, Schausberger A, Winter HH. Rheol Acta 1990;29:400-8]. Published rheological parameters appropriate for polyethylene were used in the calculations. It was found that Eta0 depended mostly on M_w, but it also significantly depended on the extent of high-MW polydispersity M_z/M_w. A revision to the fundamental MW dependency of Eta0 was proposed to compensate for this polydispersity effect. To offset the polymer polydispersity differences, we propose a new MW average (M_HV or M_x with x = 1.5) to replace M_w in the historical rheological power-law equation of Eta0 ∝ M_w^a, where the literature value of exponent “a” ranges from 3.2 to 3.6. The use of M_HV instead of M_w in the power-law equation made the calculated Eta0 independent of the sample high-MW polydispersity. With the removal of the complication from polydispersity effect, the new Eta0 power law can now provide a more robust base for studying polymer long-chain branching (LCB). A new LCB index is thus proposed based on this new melt-viscosity power law. The values of M_HV in the new power law can be calculated for polymer samples from the conventional gel permeation chromatographic (GPC) slice data.     

Synthesis of star polymethyl acrylate by SET-LRP at ambient temperature

The star-shaped polymethyl acrylate (PMA) was synthesized by single electron transfer living radical polymerization (SET-LRP) at 30 °C in dimethyl sulfoxide, using 2,2-dibromomethyl-1,3-dibromopropane as the multifunctional initiator, Cu⁰ powder and tris-(2-dimethylamino ethyl)amine (Me₆-TREN) as catalyst. The structure of polymer was analyzed by ¹H NMR, and the results showed that the star-shaped PMA had perfect chain ends (–Br) retention. In addition, the polymerization proceeded smoothly and the time dependence of ln([M]₀/[M]) was linear, which could indicate a first order propagation rate with respect to both radicals and monomer concentration, the polymerization was the living polymerization. The M _n and M _w/M _n of polymer were being measured by Gel Permeation Chromatography. The k _p^app = 0.0367 h⁻¹ and the conversion was 36.3% at 16 h, meanwhile the M _n^GPC of the polymer was 13,300 and the M _w/M _n was 1.40.     

A molar mass induced transition in the yielding properties of linear polyethylene

The mechanism of tensile deformation in a semi-crystalline polymer like PE changes at four critical points which can be associated with (A) the onset of single slip-processes, (B) a turnover into a collective activity of slips, (C) the beginning of a disaggregation of the crystal blocks, followed by fibril formation, and (D) the onset of chain disentangling. Studies of the deformation behavior of series of LPEs with increasing molar mass show an abrupt rise of the critical strains ε_H(C) and ε_H(D) at a molar mass of M_t≈(1 5-2)×10⁵ g mol⁻¹. As indicated by dynamic mechanical tests the rise originates from a change in the mobility and compliance of the amorphous regions, which are melt-like for M_w>M_t, but reduced for M_w<M_t. As a consequence, both, crystal block disaggregation and chain disentangling, set in at lower strains.     

Synthesis and properties of new electroluminescent polymers possessing both hole and electron-transporting units in the main chain

New EL polymers possessing both hole and electron-transporting units in the main chain are synthesized. The polymer prepared by palladium catalyzed Heck reaction of 10 and 15 show a large weight average molecular weight (M_w) (25,000) and small polydispersity index (PDI) (1.2). The oligomers synthesized by Wittig condensation have M_w of 4000 and PDI of 1.8. All the polymer and oligomers synthesized exhibit remarkable thermal stability with high decomposition temperature and high T_g as determined by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC) under nitrogen atmosphere. The EL emission maximum peaks of the materials prepared are in the range of 535-560 nm corresponding to green-yellowish-green. Among the three electron-transporting moieties, the 1,3,4-oxadiazole unit shows the best electron injection and transporting property.     

A technique to infer structural information for low level long chain branched polyethylenes

A technique is proposed to relate the weight average molecular weight of linear chains, M_wBL, in low level long chain branched polyethylenes to their linear viscoelastic data. The new method is based on a previously reported empirical technique [Macromolecules 33 (2000) 7481] and was developed through the use of basic molecular theories. The new technique was applied to model systems whose linear viscoelastic properties were simulated using the molecular model of Milner et al. [Macromolecules 31 (1998) 9345] and to long chain branched metallocene polyethylenes. It is applicable to branched polyethylenes with low levels of long chain branching. In the case of a branched metallocene polyethylene, the structural parameter, M_wBL, inferred from the rheological data together with the GPC data such as M_w or M_n of the sample describes all aspects of the structure of the polymer. In the case of highly branched polymers, a possible modification of the technique is also proposed.     

Molecular weight and temperature dependence of self-diffusion coefficients in polyethylene and polystyrene melts investigated using a modified n.m.r. field-gradient technique

Self-difusion in polyethylene and polystyrene melts has been investigated using a modified n.m.r. field-gradient technique. In both polymer examples, self-diffusion coefficients were found to be proportional to M^?2.0±0.1_w above and below the critical molecular weight M_c. The observation of undeuterated chains in deuterated matrices of varying chain length indicates only relatively weak matrix effects. The influence of internal field gradients, which have previously been reported for polyethylene melts, is demonstrated and discussed. Certain peculiarities have been observed in the temperature dependences of the diffusion coefficients.     

Miniemulsion vs. conventional emulsion polymerization for pressure-sensitive adhesives production

A systematic study of the production of poly(2-ethyl hexyl acrylate/methyl methacrylate/acrylic acid) pressure-sensitive adhesives (PSAs) via conventional emulsion and miniemulsion polymerization was carried out in order to discern and compare the influence of copolymer composition, chain transfer agent (CTA) and surfactant concentrations on the kinetics and microstructure of the resulting adhesive films. The results showed that miniemulsion polymerization proceeded solely under droplet nucleation for a certain set of initial reaction conditions. The miniemulsion-based latexes presented a polymer microstructure that was different to that found in latexes produced by conventional emulsion polymerization, as observed in the frequency master curves and PSA performance analysis. Batch miniemulsion polymerization was able to produce films with entanglement densities (M_w/M_e) from 2 up to 11, which were strongly correlated with loop tack, peel strength and shear strength. Conversely, under the same reaction conditions, batch conventional emulsion polymerization was only able to produce M_w/M_e ratios below 2.