The impact of thermal history on the kinetic response of thin thermoresponsive diblock copolymer poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate), abbreviated as PMEO2MA-b-POEGMA300, films is investigated by in situ neutron reflectivity. The PMEO2MA and POEGMA300 blocks are both thermoresponsive polymers with a lower critical solution temperature. Their transition temperatures (TTs) are around 25 °C (TT1, PMEO2MA) and 60 °C (TT2, POEGMA300). Thus, by applying different temperature protocols (20 to 60 or 20 to 40 to 60 °C), the PMEO2MA-b-POEGMA300 thin films experience different thermal histories: the first protocol directly switches from a swollen to a collapsed state, whereas the second one switches first from a swollen to a semicollapsed and finally to a collapsed state. Although the applied thermal histories differ, the response and final state of the collapsed films are very close to each other. After the thermal stimulus, both films present a complicated response composed of an initial shrinkage, followed by a rearrangement. Interestingly, a subsequent reswelling of the collapsed film is only observed in the case of having applied a thermal stimulus of 20 to 40 °C. The normalized film thickness and the D2O amount of each layer in the PMEO2MA-b-POEGMA300 films are consistent at the end of the two different thermal stimuli. Hence, it can be concluded that the thermal history does not influence the final state of the PMEO2MA-b-POEGMA300 films upon heating. Based on this property, these thin films are especially suitable for the temperature switches on the nanoscale, which may experience different thermal histories.
The effect of chain architecture on the swelling and thermal response of thin films obtained from an amphiphilic three-arm star-shaped thermo-responsive block copolymer poly(methoxy diethylene glycol acrylate)-block-polystyrene ((PMDEGA-b-PS)) is investigated by in situ neutron reflectivity (NR) measurements. The PMDEGA and PS blocks are micro-phase separated with randomly distributed PS nanodomains. The (PMDEGA-b-PS) films show a transition temperature (TT) at 33 °C in white light interferometry. The swelling capability of the (PMDEGA-b-PS) films in a DO vapor atmosphere is better than that of films from linear PS-b-PMDEGA-b-PS triblock copolymers, which can be attributed to the hydrophilic end groups and limited size of the PS blocks in (PMDEGA-b-PS). However, the swelling kinetics of the as-prepared (PMDEGA-b-PS) films and the response of the swollen film to a temperature change above the TT are significantly slower than that in the PS-b-PMDEGA-b-PS films, which may be related to the conformation restriction by the star-shape. Unlike in the PS-b-PMDEGA-b-PS films, the amount of residual DO in the collapsed (PMDEGA-b-PS) films depends on the final temperature. It decreases from (9.7 ± 0.3)% to (7.0 ± 0.3)% or (6.0 ± 0.3)% when the final temperatures are set to 35 °C, 45 °C and 50 °C, respectively. This temperature-dependent reduction of embedded DO originates from the hindrance of chain conformation from the star-shaped chain architecture.
The linear shrinkage behavior in thermoresponsive diblock copolymer films and its potential application in temperature sensors are investigated. The copolymer is composed of two thermoresponsive blocks with different transition temperatures (TTs): di(ethylene glycol) methyl ether methacrylate (MEO 2 MA; TT 1 = 25 °C) and poly(ethylene glycol) methyl ether methacrylate (OEGMA 300 ; TT 2 = 60 °C) with a molar ratio of 1:1. Aqueous solutions of PMEO 2 MA-b-POEGMA 300 show a three-stage transition upon heating as seen with optical transmittance and small-angle X-ray scattering: dissolution (T < TT 1 ), selfassembled micelles with core−shell structure (TT 1 < T < TT 2 ), and aggregation of collapsed micelles (T > TT 2 ). Due to the restrictions in the polymer chain arrangement introduced by the solid Si substrate, spin-coated PMEO 2 MA-b-POEGMA 300 films exhibit an entirely different internal structure and transition behavior. Neutron reflectivity shows the absence of an ordered structure normal to the Si substrate in as-prepared PMEO 2 MA-b-POEGMA 300 films. After exposure to D 2 O vapor for 3 h and then increasing the temperature above its TT 1 and TT 2 , the ordered structure is still not observed. Only a D 2 O enrichment layer is formed close to the hydrophilic Si substrate. Such PMEO 2 MA-b-POEGMA 300 films show a linear shrinkage between TT 1 and TT 2 in a D 2 O vapor atmosphere. This special behavior can be attributed to the synergistic effect between the restrained collapse of the PMEO 2 MA blocks by the still swollen POEGMA 300 blocks and the impedance of chain arrangement by the Si substrate. Based on this unique behavior, spin-coated PMEO 2 MA-b-POEGMA 300 films are further prepared into a temperature sensor by implementing Ag electrodes. Its resistance decreases linearly with temperature between TT 1 and TT 2 .
The kinetic rehydration of thin di-block copolymer poly(diethylene glycol monomethyl ether methacrylate)-block-poly(poly(ethylene glycol) methyl ether methacrylate) (PO 2 -b-PO 300 ) films containing two thermoresponsive components is probed by in situ neutron reflectivity (NR) with different thermal stimuli in the D 2 O vapor atmosphere. The transition temperatures (TTs) of PO 2 and PO 300 blocks are 25 and 60 °C, respectively. After the one-step stimulus (rapid decrease in temperature from 60 to 20 °C), the film directly switches from a collapsed to a fully swollen state. The rehydration process is divided into four steps: (a) D 2 O condensation, (b) D 2 O absorption, (c) D 2 O evaporation, and (d) film reswelling. However, the film presents a different rehydration behavior when the thermal stimulus is separated into two smaller steps (first decrease from 60 to 40 °C and then to 20 °C). The film first switches from a collapsed to a semiswollen state caused by the rehydrated PO 300 blocks after the first step of thermal stimulus (60 to 40 °C) and then to a swollen state induced by the rehydrated PO 2 blocks after the second step (40 to 20 °C). Thus, the kinetic responses are distinct from that after the one-step thermal stimulus. Both the time and extent of condensation as well as evaporation processes are significantly reduced in these two smaller steps. However, the final states of the rehydrated PO 2 -b-PO 300 films are basically identical irrespective of the applied thermal stimulus. Thus, the final state of thermoresponsive di-block copolymer films is not affected by the external thermal stimuli, which is beneficial for the design and preparation of sensors or switches based on thermoresponsive polymer films.
Thermo-responsive random copolymer poly (2-(2-methoxyethoxy) ethoxyethyl methacrylate-co-ethylene glycol methacrylate) P(MEO2MA-co-EGMA) was investigated in thin film. By spin-coating, the obtained film thickness varied from 9 nm to 97 nm, which shows a linear relationship with tetrahydrofuran solution concentration. The swelling and transition behavior of P(MEO2MA-co-EGMA) films were monitored by white-light interferometry under water vapor atmosphere. It is observed that the film rapidly swelled in the first 90 min. Afterwards it reached an equilibrium state. The film thickness did not show a prominent increase by further prolonging the swelling time. In addition, the swelling capability of P(MEO2MA-co-EGMA) films was related to the film thickness. The thicker film possessed less swelling capability. Unlike the transition behavior in aqueous solution, P(MEO2MA-co-EGMA) films showed a much broader transition region, which might be related to the influence of Si substrate.
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