Reversible thickness control of polymer thin films containing photoreactive coumarin derivative units

The reversible control of the thickness of polymer thin films was investigated using (meth)acrylic polymers containing photoreactive coumarin derivative units in the side chain. Coumarin derivative units underwent dimerization and the reverse-dimerization by photoirradiation and were used as a reversible cross-linking point. The homopolymer of 7-methacryloyloxy-4-methylcoumarin (T_g = 194 °C) did not cause changes in film thickness after photoreactions. The homopolymer of 7-(2′-acryloyloxyethoxy)-4-methylcoumarin (AEMC) (T_g = 89 °C) decreased 19% of film thickness by photodimerization and 73% of the decreased thickness was recovered after the reverse-dimerization and the subsequent thermal annealing at 130 °C. The reverse-dimerization of the copolymer of AEMC and n-butyl acrylate (AEMC content = 19 mol%, T_g = 11 °C) resulted in 53% of recovery from the decreased film thickness without annealing. The mobility of polymer main-chain was revealed to be essential factor to change film thickness by photoreactions. Photodimerization of coumarin derivative units in low glass transition temperature (T_g) tended to proceed faster than in high T_g polymers and resulted in larger decrease in film thickness.     

Physical aging of layered glassy polymer films via gas permeability tracking

The physical aging of polymers in confined environments has been an area of intensive study in recent times. The rate of physical aging in thin films of many polymers used in gas separation membranes is dependent on film thickness and accelerated relative to bulk. In this study, the physical aging of polymer films with alternating glassy polysulfone and rubbery polyolefin layers was monitored by measuring the gas permeability of O₂ and N₂ as a function of aging time at 35 °C. The alternating layer structures were formed by a melt co-extrusion process. The polysulfone layers have thicknesses ranging from 185 to 400 nm, and the overall thicknesses of the films are on the order of 80-120 μm. The aging of freestanding thin films of polysulfone is rapid and exhibits clear thickness dependence, whereas the aging of multilayered films was observed to be similar to bulk and showed no dependence on layer thickness. At 1000 h of aging time, a 400 nm freestanding PSF film decreased in O₂ permeability by 35%, whereas on average the bulk and multilayered films only experienced a decline of 10-15%. A slight increase in O₂/N₂ selectivity for the multilayered films was observed over the course of aging.     

Effects of film thickness on moisture sorption, glass transition temperature and morphology of poly(chloro-p-xylylene) film

Moisture sorption, glass transition temperature (T_g) and morphology of ultra thin poly(chloro-p-xylylene) (parylene-C) are greatly influenced by geometrical confinement effects. For film <50 nm, the equilibrium moisture saturation is a decreasing function of film thickness. However, the T_g of film <50 nm is about 10 °C higher than thicker films. The above phenomena are attributed to the effect of geometrical confinement on the thermal properties and the morphology of parylene-C film. Surface confinement results in an increased in T_g, but a decreased in crystallinity for films <50 nm. In this study, we show that the increase in moisture sorption for parylene-C films <50 nm is dominated by the crystallinity variation.     

In situ monitoring of thermal transitions in thin polymeric films via optical interferometry

A novel, low-cost, rapid, accurate, non-invasive and high throughput method based on the principles of Optical Interferometry (OPTI method) has been developed and applied for the in situ monitoring in one simple run of first (melting) and second (glass transition) order transitions as well as of the thermally induced decomposition of various thin polymeric films spin coated on flat reflective substrates (untreated silicon wafers). The new method has been applied successfully for measuring the glass transition, melting and decomposition temperatures of six commercially available polymers [poly(methyl methacrylate) (PMMA), poly(2-hydroxyethyl methacrylate), (PHEMA), poly(vinyl acetate-co-crotonic acid), (PVACA), poly(vinyl pyrrolidone) (PVP), poly(vinyl chloride-co-vinyl acetate) (PVCVA) and crystalline poly(vinylidene fluoride-co-hexafluoropropylene) (PVFHP)] of known T_gs or T_ms. The recorded interferometric signals were identified and characteristic signal patterns were qualitatively correlated to specific transitions. The monitoring of first and second order transitions in thin polymeric films is based on detectable differentiations of the total energy of a fixed wavelength laser beam incident almost vertically (angle of incidence <5°) onto a thin polymeric film spin coated on a flat reflective substrate. These differentiations are caused by film thickness and/or refractive index changes of the polymeric film both resulted from the significant change of the polymer's free volume taking place on the transitions. For film thicknesses over approx. 200-250 nm, the T_g or T_m of the polymeric films measured with the OPTI method were in excellent agreement with the corresponding values of the polymer, measured by DSC. An investigation on the trends of the T_g of PHEMA and PMMA films in a wide thickness range (30-1735 nm) was also carried out. Ultra-thin (∼30 nm) films of PMMA and PHEMA showed significant increase in their T_g values by approx. 30 °C upon comparing to their corresponding bulk T_gs. This behavior was attributed to an enhanced polymer-surface interaction through hydrogen bonding and/or to changes in the tacticity of the polymer.     

Influence of previous history on physical aging in thin glassy polymer films as gas separation membranes

The physical aging behavior of thin glassy polysulfone (PSF) films (∼125 nm) with different previous histories was tracked using gas permeability measurements. The initial states of these materials were modulated by thermal annealing at fixed temperatures below the glass transition or by exposure to high pressure (800 psig (56.2 bara)) CO₂ for various times. Regardless of the previous history, the nature of the aging response in these samples was consistent with the aging behavior of an untreated film that was freshly quenched from above T_g, i.e., permeability decreased and pure gas selectivity increased with aging time. However, the extent of aging-induced changes in transport properties of these materials depended strongly on previous history. The aging behavior was described using Struik’s aging model by allowing the initial conditions to depend on each sample’s previous history.     

Synthesis, characterization and optical properties of cross-linkable poly(phthalazinone ether ketone sulfone)

Cross-linkable poly(phthalazinone ether ketone sulfone) bearing tetrafluorostyrene groups (PPEKS-FSt) has been prepared by copolycondensation reaction for optical waveguide applications. The resulting amorphous polymer exhibits good solubility in some common polar organic solvents (e.g., N,N′-dimethylacetamide, N-methyl-2-pyrrolidinone, chloroform) at room temperature, and can be easily spin-coated into thin films with good optical quality. The glass transition temperature (T_g) and the temperature of 1% weight loss (1% T_d) are 261 °C and 494 °C, respectively, which could be further increased by 31 °C and 14 °C upon thermal cross-linking. The cross-linked polymer thin films exhibit high refractive index (∼1.65, TE mode), high thermo-optic coefficient value (dn/dT) (−1.455 × 10⁻⁴/°C, TE mode), low optical loss (less than 0.24 dB/cm at 1310 nm) and relatively low birefringence (∼0.007).     

Methacrylate-based polymer films useful in lithographic applications exhibit different glass transition temperature-confinement effects at high and low molecular weight

The effects of confinement on polymer films are important in applications related to photoresists. To optimize resolution, methacrylate polymers used in photoresists are often low molecular weight (MW). We use ellipsometry and fluorescence to study how the glass transition temperature (T_g) is affected by confinement in silica-supported films of low and high MW poly(1-ethylcyclopentyl methacrylate) (PECPMA) and poly(methyl methacrylate) (PMMA). With decreasing nanoscale thickness, T_g is nearly invariant for high MW (M_n = 22.5, 188 and 297 kg/mol) PECPMA but decreases for low MW PECPMA, with T_g – T_g,bulk = −7 to 8 °C in a 27-nm-thick film (M_n = 4.1 kg/mol) via ellipsometry and −15 °C in a 17-nm-thick film (M_n = 4.9 kg/mol) via fluorescence. Fluorescence studies using a 20-nm-thick dye-labeled layer in multilayer, bulk PECPMA films reveal a much greater perturbation to T_g in the free-surface layer for low MW PECPMA, which propagates tens of nanometers into the film. The effect of MW in single-layer monodisperse PMMA films is even more striking; T_g increases with confinement for high MW but decreases for low MW, with T_g – T_g,bulk = 9 °C in a 12-nm-thick film (nominal MW = 509 kg/mol) and −16 °C in a 17-nm-thick film (nominal MW = 3.3 kg/mol). The strong influence of MW on confinement effects in PECPMA and PMMA is in contrast to the previously reported invariance of the effect with MW in supported polystyrene films, reconfirmed here.     

Physical aging of ultrathin glassy polymer films tracked by gas permeability

Membrane-based separations play a key role in energy conservation and reducing greenhouse gas emissions by providing low energy routes for a wide variety of industrially-important separations. For reasons not completely understood, membrane permeability changes with time, due to physical aging, and the rate of permeability change can become orders of magnitude faster in films thinner than one micron. The gas transport properties and physical aging behavior of free-standing glassy polysulfone and Matrimid^® films as thin as 18 nm are presented. Physical aging persists in glassy films approaching the length scale of individual polymer coils. The films studied ranged from 18–550 nm thick. They exhibited reductions in gas permeability, some more than 50%, after 1000 h of aging at 35 °C, and increases in selectivity. The properties of these ultrathin films deviate dramatically from bulk behavior, and the nature of these deviations is consistent with enhanced mobility and reduced T_g in ultrathin films. The Struik physical aging model was extended to account for the influence of film thickness on aging rate, and it was shown to adequately describe the aging data.     

Polyimides from 2,3,3′,4′-biphenyltetracarboxylic dianhydride and aromatic diamines

A series of new polyimides were prepared from the reaction of 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA) with various aromatic diamines. The properties of the a-BPDA polyimides were compared with those of polyimides prepared from the reaction of 3,3′,4,4′-biphenyltetracarboxylic dianhydride (s-BPDA) with the same aromatic diamines. Films of the a-BPDA polyimides had higher glass transition temperatures (T_gs) and less color than the corresponding s-BPDA polyimide films. Light transmission at 500 nm, solar absorptivity, and thermal emissivity were determined on certain films. Films of similar polyimides based upon a-BPDA and s-BPDA containing meta linkages and others containing para linkages were each cured at 250, 300, and 350 °C. The films were characterized primarily by T_g, color, optical transparency, tensile properties, dynamic mechanical thermal analysis, and coefficient of thermal expansion. The a-BPDA meta linked polyimide films had tensile strengths and moduli higher than films of the a-BPDA para linked polyimide. The same phenomenon was not observed for the s-BPDA meta and para linked polyimides. The chemistry, mechanical, and physical properties of the polymers and films are discussed.     

High thermal stable thermoplastic-thermosetting polyimide film by use of asymmetric dianhydride (a-BPDA)

In order to obtain thermoplastic (before curing) and thermosetting (after curing) polyimides with high T_g for adhesive film, we prepared novel polyimides having phenylethynyl group in the side chain (44% of concentration of curing group) from asymmetric 2,3,3′,4′-biphenyltetracarboxylic dianhydride (a-BPDA), 3,4′-oxydianiline (3,4′-ODA) or 1,3-bis(4-aminophenoxy)benzene (1,3,4-APB) or 1,3-bis(3-aminophenoxy)benzene (1,3,3-APB), and 2,4-diamino-1-(4-phenylethynylphenoxy)benzene (mPDAp). Among three kinds of polymer, uncured polyimide of a-BPDA/1,3,4-APB; mPDAp had rather high T_g (265 °C, DMA) and thermoplasticity (E′ drop>10³ at T_g). After curing reaction of phenylethynyl group, the T_g of the polyimide was increased dramatically (364 °C, DMA). The polyimide derived from 1,3,4-APB having less concentration of curing group (20%) was also prepared to improve further film flexibility and toughness.     

Synthesis and electroluminescence properties of a novel tetraphenylsilane-oxadiazole-diphenyl(para-tolyl)amine polymer

A novel triarylaminooxadiazole-containing tetraphenylsilane light-emitting polymer (PTOA) has been synthesized. Excellent thermal stability was observed due to the presence of a rigid tetraphenylsilane-based polymer backbone (T_g = 218 °C, T_d = 373 °C). In solution, PTOA shows photoluminescence (PL) with an emission maximum at 426 nm, which is attributed to the light-emitting unit of the triarylaminooxadiazole group. In solid film, the emission maximum of PL is observed at 458 nm, a 32 nm red-shift from the PL in solution. The solvatochromic effect and excimer formed in the solid film are responsible for the red-shifting and broadening of the PL emission band. The PL stability and morphology of the PTOA solid film were further investigated by thermal annealing at elevated temperatures. No significant difference in the PL spectra or morphology was observed between a pristine sample and a repeatedly thermally annealed film (at 200 °C). PTOA-based PLED shows EL with a main peak at 458 nm accompanied by a shoulder at around 530 nm. The light emission from electromer or electroplex leads to a broadening of the EL spectra (400-650 nm), which corresponds to the interaction between the oxadiazole and diphenyl(4-tolyl)amine groups in different polymer segments or chains. A sky blue emission (Commission Internationale de L'Eclairage (CIE_x,y) coordinates (0.20,0.23)) was obtained for PTOA-based PLED. The brightness and efficiency of the PLED can be as high as 248 cd/m² and 0.54 cd/A, respectively. The EL of PTOA-based PLED has been further improved by blending the PTOA with poly(n-vinylcarbazole) (PVK) in different concentrations. The effects of concentration on the PL and EL were studied for the PTOA-PVK composite film-based PLEDs.     

Thermally stable NLO poly(amide-imide)s via sequential self-repetitive reaction

A series of thermally stable side-chain second-order nonlinear optical (NLO) poly(amide-imide)s via sequential self-repetitive reaction (SSRR) have been developed. This SSRR is based on carbodiimide (CDI) chemistry. Three difunctional azo chromophores (DR19, NDPD and DNDA) were, respectively, reacted with excessive amount of 4,4′-methylene-diphenylisocyanate (MDI) to form poly-CDI, and subsequently trimellitic anhydride (TMA) was added to obtain an intermediate, poly(N-acylurea). Poly(N-acylurea) exhibits excellent organosolubility, which enables the fabrication of high quality optical thin films. Moreover, its moderate glass transition temperature (T_g) characteristic allows the NLO-active polymer to exhibit high poling efficiency. After in situ poling and curing process, N-acylurea moieties were converted to amide-imide structures via SSRR, and the T_gs of the polymers were elevated significantly up to 70 °C higher than that of the poly-CDI sample. Electro-optical coefficients, r₃₃ of about 5.2-25.2 pm/V at 830 nm were obtained. Good temporal stability (80 °C) and waveguide optical losses (3.8-6.6 dB/cm at 830 nm) were also obtained for these polymers.     

Radiochemical ageing of an amine cured epoxy network. Part I: change of physical properties

An aromatic rich, amine cured epoxy network (initial glass transition temperature 250 °C), was irradiated in air (pressure 0.22 MPa), at 30 and 120 °C, by gamma rays with two dose rates 2 and 20 kGy/h, for doses upto 70 MGy. The following characteristics were recorded, thickness of oxidised layer (TOL) from IR microspectrophotometry, flexural strength σ_R, toughness K_IC and glass transition temperature T_g. σ_R decreases from 120 MPa to about 40 MPa in the most degraded samples. This decrease is sharply linked to TOL showing the key role of the oxidised layer in crack initiation. K_IC decreases from 0.7 to 0.55 MPa m^1/2. Data are too much scattered to allow a kinetic study but it appears that, in the early period of exposure, K_IC decreases more rapidly at 120 °C than at 30 °C. T_g decreases from 250 to 140 °C in the most degraded samples, and the decrease is faster at 30 °C than at 120 °C. The decrease of T_g is attributed to a predominant chain scission process. The decrease of K_IC can be attributed to a combination of chain scission and physical ageing or chain scission and crosslinking. A relationship between T_g and the number of chain scissions, derived from the Di Marzio's theory, is proposed.     

Synthesis and characterization of perfluorocyclobutyl (PFCB) polymers containing pendent phenylphosphine oxide

Three phenylphosphine oxide (PPO) containing trifluorovinyl aryl ether monomers were synthesized and polymerized via thermal cyclodimerization affording perfluorocyclobutyl (PFCB) polymers containing PPO pendent groups. The new polymers exhibited moderate to high glass transition temperatures (T_g=145-217 °C) and good thermal stability in nitrogen (5% weight loss, T_d>402 °C). Copolymerization with traditional PFCB forming monomers such as 4,4′-(trifluorovinyloxy)biphenyl resulted in film forming transparent thermoplastic copolymers with improved solubility and further enhanced thermal stability. Semi-fluorinated PPO containing polymers of this type may find potential application as space environment durable materials.     

Synthesis of nitrile and benzoyl substituted poly(biphenylene oxide)s via nitro displacement reaction

A series of substituted poly(biphenylene oxide)s (PBPOs) was synthesized via nucleophilic nitro displacement reactions. High molecular weight PBPO's with nitrile groups were effectively synthesized from the polymerization of A-B type monomers with K₂CO₃ as a base in N-methyl-2-pyrrolidinone (NMP) at 140 °C. The polymers are completely amorphous, soluble in polar aprotic solvents, and formed flexible films on solution casting. Para-linked PBPO with nitrile groups showed excellent thermal properties such as high 5% weight loss temperature above 530 °C and T_g at 241 °C which is higher than those of commercially available PPO™ (T_g=210 °C). The pendent nitrile groups of PBPO were easily transformed to carboxylic acid groups by acidic hydrolysis.     

Synthesis and characterization of polyimides and co-polyimides having pendant benzoic acid moiety

A novel diamine monomer, 2,4-diamino-4′-carboxy diphenyl ether had been synthesized. Several polyimides were prepared by reacting this diamine with commercially available dianhydrides, such as benzophenone tetracarboxylic acid dianhydride (BTDA), 4,4′-bis{hexafluoroisopropylidene bis (phthalic anhydride)}(6-FDA), oxydiphthalic anhydride (ODPA) and 3,3′,4,4′-biphenyltetracarboxylic acid dianhydride (BPDA). Furthermore, copolymers from the resulting diamine and oxydianiline (ODA) with 6 FDA were also synthesized. The inherent viscosities of the polymers were 0.42-0.67 dl g⁻¹. The polymers have good solubility in polar aprotic solvents, high thermal stability up to 410 °C in nitrogen and high glass transition temperatures (T_g) ranging from 260-330 °C. These polymers formed tough flexible films by solution casting.     

Highly ion conductive flexible films composed of network polymers based on polymerizable ionic liquids

Ionic liquid-type polymer brushes having different hydrocarbon (HC) chain lengths between polymerizable group and imidazolium ring were synthesized. When the carbon number of HC chain was 6, the ionic liquid-type polymer brush exhibited the highest ionic conductivity of 1.37×10⁻⁴ S cm⁻¹ at 30 °C, reflecting low T_g of −60 °C. Moreover, for the first time, we succeeded in obtaining transparent and flexible films without considerable decrease in the ionic conductivity as compared with that of corresponding monomers by using suitable cross-linkers. The most ion conductive (1.1×10⁻⁴ S cm⁻¹ at 30 °C) film was obtained when tetra(ethylene glycol)diacrylate was used 0.5 mol% to ionic liquid monomer as the cross-linker. This film is one of excellent conductive films among single-ion conductive materials.     

Isothermal physical aging of thin PMMA films near the glass transition temperature

Isothermal physical aging and the glass transition temperature (T _g) of PMMA thin films were investigated by means of differential scanning calorimetry (DSC). Freestanding thin films of different molecular weights (M _w = 120,000, 350,000, 996,000 g/mol) and film thicknesses (40–667 nm) were obtained by spin coating onto a silicon wafer substrate and then releasing the coated film using a water floating technique. The thin films were stacked in a DSC pan and isothermally aged for different aging times (t _a = 1 and 12 h) and aging temperatures (T _a = 105, 110, and 115 °C) below but near T _g. Enthalpy relaxation (ΔH _Relax), resulting from the isothermal physical aging, initially increased with increasing ΔT (T _g − T _a, driving force of aging), reached a maximum value, and then decreased with further increase in ΔT. Below ~100 nm film thickness, ΔH _Relax of samples aged near their T _g (i.e., T _a = 110 and 115 °C) decreased with decreasing film thickness, indicating the suppression of physical aging. Up to 9.9 °C depression in T _g was observed for thinner films (~40 nm), when compared to the thicker films (~660 nm) in this study. The decrease in ΔH _Relax with decreasing film thickness at a given T _a appears to be associated with the reduction in T _g.     

Facile tailoring of thermal transition temperatures of epoxy shape memory polymers

A critical parameter for a shape memory polymer (SMP) lies in its shape memory transition temperature. For an amorphous SMP polymer, it is highly desirable to develop methods to tailor its T_g, which corresponds to its shape memory transition temperature. Starting with an amine cured aromatic epoxy system, epoxy polymers were synthesized by either reducing the crosslink density or introducing flexible aliphatic epoxy chains. The thermal and thermomechanical properties of these epoxy polymers were characterized by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). All the crosslinked epoxy polymers with T_g's above room temperature were found to possess shape memory properties. Overall, our approach represents a facile method to precisely tune the T_g of epoxy SMP polymers ranging from room temperature to 89 °C.     

High glass transition and thermal stability of new pyridine-containing polyimides: Effect of protonation on fluorescence

A new diamine monomer containing heterocyclic pyridine and triphenylamine groups, 4-(4,4′-diaminotriphenylamine)-2,6-bis(4-methylphenyl)pyridine (4), was synthesized by Chichibabin and nucleophilic fluoro-displacement reactions. The diamine was used to prepare a series of novel polyimides via polycondensation with various aromatic tetracarboxylic dianhydrides in N-methyl-2-pyrrolidinone. The polyimide 4a derived from the diamine 4 with 4,4′-hexafluoroisopropylidenediphthalic anhydride and having high T_g (313 °C), mechanical, and thermal properties was soluble in various organic solvents, such as N-methyl-2-pyrrolidinone, N,N-dimethylacetamide, N,N-dimethylformamide, pyridine, chloroform, tetrahydrofuran, at room temperature. The polyimide (4a) could be cast into a self-standing film from DMAc solution and was thermally converted into tough and flexible film. The film had high tensile modulus of 2.2 GPa and exhibited excellent thermal stability in both nitrogen and air (T_d¹⁰ > 550 °C). The pristine polymer exhibited the UV-vis absorption bands in the region 240-400 nm and protonated polymer exhibited absorption in the region 390-500 nm. The protonated polymer possessed strong orange fluorescence (around 600 nm) in THF solution after protonation with acids as excited at 438 nm. The fluorescent intensity was influenced by the acid concentrations and the chemical structure of conjugated bases. The fluorescent intensity at 600 nm increased as acid concentration from a lower to a moderate concentration and decreased at higher concentrations.