Properties of a novel thermal sensitive polymer based on poly(vinyl alcohol) and its layer-by-layer...

Properties of a novel thermal sensitive polymer based

on poly(vinyl alcohol) and its layer-by-layer assembly

Hong Lu1,3, Anna Zheng1 and Huining Xiao2,3*1School of Materials Science and Engineering, Key Laboratory for Ultrafine Materials of Ministry of Education, East China University of Science &

Technology, Shanghai 200237, China2State Key Laboratory of Pulp and Paper Engineering, South China University of Technology, Guangzhou 510640, China3Department of Chemical Engineering, University of New Brunswick, Fredericton, New Brunswick, Canada E3B 5A3

Received 21 October 2006; Revised 12 December 2006; Accepted 12 December 2006

Structurally modified poly(vinyl alcohol) (PVA) was prepared as novel thermally sensitive polymers

by partially acetalyzing and/or ionizing the commercially available PVA. Their aqueous solutions

experience completely reversible polymer aggregation and dissolution above and below the lower

critical solution temperature (LCST), respectively. The LCST of a partially acetalyzed PVA (APVA)

can be readily controlled by the degree of acetalysis or the molecular weight of the starting PVA.

Introduction of a small amount of cationic group onto the APVA backbone increases the LCST

obviously, while the LCST is highly sensitive to NaCl concentration. ThenAPVA and cationic APVA

multilayers are assembled on rayon to make a thermal responsive fiber. The atomic force microscopy

(AFM) images of the surface reveal the increment of roughness stimulated by temperature.

Copyright # 2007 John Wiley & Sons, Ltd.

KEYWORDS: temperature responsive; poly(vinyl alcohol); self-assembly; atomic force microscopy (AFM); fibers

INTRODUCTION

Water-soluble thermal responsive polymers undergo fast,

reversible volume changes around their lower critical

solution temperature (LCST).1–6 Below LCST, the free

polymer chains are soluble in water and exist in an extended

random coil conformation that is fully hydrated. On the

contrary, above LCST, the chains hydrophobically fold as a

result of dehydration and assemble to form a phase

separating state. These polymers can be formed as gels,

particles, micelles, and capsules, etc. Therefore, they are

potentially very attractive for many engineering appli-

cations, such as tissues,7,8 controlled drug deliverer,9–12

and catalyst carrier,13–15 etc.

In this research, partially acetalyzed poly(vinyl alcohol)

(APVA) and cationic partially acetalyzed poly(vinyl alcohol)

(CAPVA) were used as novel thermal responsive polymers

with LCST because their hydrophilicity was reduced by the

reaction between PVA and acetaldehyde. It is widely known

that the poly(vinyl alcohol) (PVA) is a low toxicity16 and

relatively inexpensive polymer which has been produced on

a large scale commercially. In addition to the wide range

of industrial applications, PVA is also used in a variety of

biomedical applications, such as in the manufacture of

artificial kidney and pancreas,17–19 cell encapsulation,20–22

artificial articular cartilage,23 drug delivery,24–27 nerve

cuffs,28 etc. The minimized protein adsorption and biocom-

patibility of PVA are the main attractions for the popular use

of PVA as a biomaterial. APVA was first prepared by

Christova et al.29 from the reaction between PVA and

acetaldehyde in acid solution. However, there were very

limited publications, further investigating the properties of

APVA, and cationic-modified APVA has not yet been

reported. For CAPVA, not only the phase transition

temperature, strength, and sensitivity can be modulated,

but also ionic strength sensitivity is introduced by the

incorporating charges.

Cellulose is the most abundant natural biopolymer, and its

derivatives have many important applications in fiber,

paper, and paint industries. A variety of monomers were

grafted onto cellulose by different techniques to control

properties, such as hydrophobicity, adhesivity, selectivity,

drug delivery, wettability, and thermosensitivity.30–32 Com-

pared to grafting polymerization, the layer-by-layer (LbL)

method is a particularly simple approach to produce thin

films of controlled structure and chemical architecture for a

variety of applications, such as optical devices, membranes,

or biologically active coatings.33–35 In LbL, polymers

sequentially deposit on a substrate, where strong electro-

static attraction or hydrogen bonds occur between the

surface and polymers in the solution. The adsorption of

cationic polymers on cellulose fiber is practically possible

POLYMERS FOR ADVANCED TECHNOLOGIES

Polym. Adv. Technol. 2007; 18: 335–345

Published online 26 February 2007 in Wiley InterScience (www.interscience.wiley.com) DOI: 10.1002/pat.891

*Correspondence to: H. Xiao, Department of Chemical Engineer-ing, University of New Brunswick, Fredericton, NB, Canada E3B5A3.E-mail: [email protected]

Copyright # 2007 John Wiley & Sons, Ltd.

because of its negatively charged surface. In this work, a

thermally sensitive fiber wasmade by assembling APVA and

CAPVA multilayers on a rayon fiber. Rayon is a cellulose

fiber with a smooth surface, compared to conventional

chemical wood fiber, because it is a manufactured fiber

composed of regenerated cellulose. The thermally sensitive

topography of a smooth surface, as observed by AFM, is

more reasonable and acceptable. The increment in roughness

or the reversible hydrophilic/hydrophobic properties of the

fiber surface, which is induced by temperature, potentially

benefits the strength development of fiber products aswell as

the retention of functional fillers or additives in conventional

or specialty papermaking processes. This is the first report

for the preparation of CAPVA and its application in

assembling of multilayers.

EXPERIMENTAL

MaterialsPVAs of three different molecular weights, 124,000–186,000,

61,000, and 13,000–23,000 g/mol, were obtained from

Aldrich. The degree of hydrolysis of PVAs was more than

98.0mol%. Acetaldehyde (FisherChemical) and glycidyltri-

methylammonium chloride (GTMAC; Fluka) were used as

received.

Synthesis of acetalyzed PVA (APVA)PVA solution was placed in a three-neck flask equipped with

a mechanical stirrer, dropping funnel and condenser. A

chosen amount of HCl was added and then acetaldehyde

was added dropwise at 158C for 30min. After the completion

of acetaldehyde feeding, themixturewas kept at 158C for 1 hr

and 308C for 24 hr. The reaction was stopped by neutraliz-

ation with 2 N NaOH aqueous solution. The solution

obtained was dialyzed for 48 hr with a dialysis membrane of

molecular weight cut-off 1000.

Synthesis of cationic APVA (CAPVA)PVA was cationized by GTMAC in a basic solution at 808Cfor 1 hr in a three-neck flask. The reactive mixture was

dialyzed for 24 hr after being neutralized by 1 N HCl

solution. Then it was acetalyzed and dialyzed following the

same procedure for synthesis of APVA.

Multilayer buildupAPVA and CAPVA (Mw¼ 124,000–128,000 g/mol) solutions

were prepared by dissolution in 5mM NaCl solution

to obtain 0.44 wt%. The silicon wafer with a 100 nm oxidized

layer and rayon fiber (from MiniFiber, Inc.) were used as the

substrates. The wafer was cleaned with 98% w/w H2SO4

solution at 608C for 4 hr. The substrate was alternately

immersed into the CAPVA solution and APVA solution for

10min each. Following each dip coating, the substrate was

rinsed thrice with D-D water and eight layers were

assembled on the substrates.

CharacterizationThe NMR spectra were recorded in DMSO-d6 or D2O at 258Cusing an Oxford 300MHz spectrometer operating at 300.13

and 75.5MHz for 1H and 13C nuclei, respectively. DMSO-d6

solutions were used to characterize the structure of APVA

andCAPVA. D2O solutions were used to calculate the degree

of acetalysis of APVA and degree of cationization of CAPVA.

Nondeuterated DMSO or H2O was used as an internal

reference. 1H, 13C, and DEPT (distortionless enhancement by

polarization transfer) experiments were performed at

standard pulse sequences. All samples were carefully dried

in a vacuum oven at 608C for 24 hr before the solutions were

made.

A HACH 2100AN turbidimeter was employed for the

LCST measurements. Samples were placed into a water bath

with temperature control. The samples were heated from 10

to 708C at a step of 2.58C. The samples were kept in the water

bath for 10min at each temperature, then taken out and dried

up quickly before inserting into the turbidimeter. The

turbidity was then recorded at each temperature.

A MultiMode Scanning Probe Microscope (Nanoscope

IIIa) with a J-scanner (maximum scan area is 125� 125mm2;

Veeco Instrument) was used to record the images of the PVA

film and was operated in contact and tapping mode. The tips

(Veeco NanoProbeTM; Digital Instruments) for contact and

tapping mode were NP-20 and TESP7, respectively. AFM

images were flattened by applying a first-order polynomial

fit to remove defects from the image, due to vertical (Z)

scanner drift.

Differential scanning calorimeter (DSC) experiments were

performed with a TA Instrument DSC Q100. All samples

were heated from 25 to 2458C at the rate of 108C/min and

held at 2458C for 5min, then cooled down to 258C at the same

rate and held at 258C for 5min. The samples were reheated

from 25 to 2458C at the rate of 108C/min again.

RESULTS

Molecular structure characterizationof APVA and CAPVAAPVA was synthesized when acetaldehyde reacts with the

adjacent hydroxyls on the PVA chain as shown in

Scheme 1a. Two kinds of acetal rings, meso and racemic,

shown in Scheme 2, resulted from isotactic and syndiotactic

PVA. Since not all of the hydroxyls reacted to form acetal

rings, APVA is essentially a copolymer of acetal ring and

vinyl alcohol. The microstructure of APVA is complicated by

the tremendous possibilities in constitutional and config-

urational variations of the copolymer.

CAPVA was synthesized by a two-step procedure. First,

the cationic monomer, GTMAC, was grafted on PVA

through the ring opening reaction of the epoxy group on

GTMAC with the hydroxyl group on PVA, as shown in

Scheme 1b. Then the cationic PVA was acetalyzed by

acetaldehyde to produce the CAPVA. NMR techniques were

applied in an attempt to assign the signals and to make the

quantitative analysis of the copolymer composition.1H spectra of APVA and CAPVA are shown in Fig. 1. As

for the symbols, A stands for APVA, the number after A is the

degree of acetalysis of PVA; C means cationization, the

number after C refers to the molar fraction of the cationic

group. Slanting letters are used to denote the carbons and

protons in the APVA and CAPVA. Resonance peaks of

methylene and methine protons a, e, e0, b, f, and f0 in the main

Copyright # 2007 John Wiley & Sons, Ltd. Polym. Adv. Technol. 2007; 18: 335–345

DOI: 10.1002/pat

336 H. Lu, A. Zheng and H. Xiao

chains of A52 appear at 1.3–1.6 and 3.77 ppm.36–37 The

resonance at 4.7 and 4.9 ppm are assigned tomethine protons

g and g0 inmeso and racemic rings, respectively, because they

connect with two oxygens directly and their resonances shift

farther downfield. The biggest peak is assigned to the g in the

meso ring because the meso ring is less strained and

predominates over the racemic rings. The proportion of the

meso rings is around 85–90mol%, found by analyzing the1H signal integrals. Consequently, the peak at 1.0–1.2 ppm

was split into two peaks corresponding to themethyl carbons

d and d0 in the meso (downfield) and racemic (upfield) rings

too. The signals of the residual OH-groups are 4.44, 4.38, 4.28,

4.18 ppm.As for the 1H-NMR spectrum of CAPVA, the single

big peak at 3.15 ppm is assigned to the methyl proton m

because it is contributed by three isolated methyl groups and

neighbors the polar ammonium chloride group. The small

peak at 4.5 ppm belongs to hydroxyl proton k on the GTMAC

side group, which can be proved by 1H-NMR of cationic

PVA. The 1H-NMR spectrum of cationic PVA is not

presented here. The signals of the proton i, l, and j are

covered by the wide peaks of methylene and methine on the

backbone.

Since 13C spectra have a much larger chemical shift range

than the 1H spectra, they are expected to showmore resolved

signals and to provide more details on configurational

microstructures. The standard 13C-NMR spectra of APVA

and CAPVA are shown in Fig. 2. The methine carbons g and

g0 of the meso and racemic rings shift further downfield to

97.6 and 91.7 ppm, respectively, because of the same reasons

mentioned for 1H-NMR. The methine carbons f and f0 of themeso and racemic rings in the main chain are shifted

downfield to 71.5–73.5 and 67.0–69.0 ppm from PVA

methines since they are in the a-position to an OR group

instead of an OH group. The resonances at 61.5–66.0 ppm are

Scheme 1. Synthesis of APVA and CAPVA. (a) Reaction of PVA and acetaldehyde. (b) Cationization

of PVA.

Scheme 2. Meso and racemic rings of APVA.


DOI: 10.1002/pat

Properties of a novel thermal sensitive polymer 337

assigned as unreacted PVAmethine carbon b. The methylene

carbons e and e0 of meso and racemic rings appear at

35.5–38.0 and 41.0–43.5 ppm, respectively. The resonances at

44.0–46.0 ppm are assigned as unreacted PVA methylene

carbon b. The methyl carbons d and d0 are observed at 21.1

and 21.3 ppm, respectively. The complicated unit sequences

of the APVA copolymer are responsible for the multiples.

The assignments of 13C-NMR spectra of CAPVA are shown

in Fig. 2b. The sharp and single peak at 53.5 ppm,

distinguished from other carbons, is the resonance of the

methyl carbon m on GTMAC side group, which is

contributed by three methyl groups. However, the reson-

ances of carbons i, l, and j are hidden in the signals of the

methylene and methine carbons of the backbone.

The DEPT spectrum of cationic PVA (CPVA) (Fig. 3) is

measured to assign the carbons i, l, and j in a simplified way

because the peaks of acetal rings are removed. The resonance

at 53.5 ppm is confirmed to be themethyl carbonm by the top

trace. Multiples at 44–47ppm in the second trace are the

methylene carbon a on the backbone of PVA, where the four

peaks are assigned to the following tacticities of PVA: rrr (at

46.2 ppm), rmrþmrr (at 45.8 ppm), mmrþmrm (at

45.4 ppm), and mmm (at 44.7 ppm).38,39 Two new and small

methylene signals were observed at 71.5 and 70.5 ppm in the

second trace also. They are reasonably assigned as the

methylene carbons i and j, respectively, since they are

directly attached to an oxygen or a nitrogen and shift farther

downfield than those methylene carbons with only methine

neighbor. The third trace had signals for methine resonance.

The peaks at 67.8, 65.8, and 63.8 ppm are assigned to be

methine carbon b in mm, mr, and rr triad sequences of PVA,

respectively. The two peaks at 64.7 and 64.5 ppm come from

the carbons j and h. The satisfied assignments of GTMAC

side group are reached by the DEPT spectrum.

Thermo-sensitivity of APVAPVA is a good water-soluble polymer at all temperature

rangeswhile the APVA shows thermo-sensitivity. To explore

Figure 1. 1H-NMR of APVA (a) and CAPVA (b) (Mw¼ 13,000–23,000).


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the phase transition behavior and the thermal sensitivity of

the APVA, turbidity curves are used to characterize the LCST

of APVA aqueous solutions. The turbidity curves of starting

PVA and APVA are plotted in Fig. 4 as a function of

temperature. At low temperature, all of the three samples are

transparent solutions and their turbidities are close to 0,

indicating that the turbidity is independent of temperature.

However, after being heated to a certain temperature near

the LCST, the transparent solutions of APVA were observed

to become cloudy, followed by a drastic increase in turbidity.

The phase separation occurs undoubtedly and can be directly

viewed from the inset of Fig. 4. On the other hand, the

starting PVA solution remains clear and the turbidity is close

to 0 during heating. In our research, the LCST is defined as

the intersection of two straight lines drawn through the

curves of turbidity at low and high temperature, respect-

ively.

A series of APVAs with different chain length and degree

of acetalysis were prepared and their LCST transitions were

investigated since the LCST transitions of thermally sensitive

polymers depend on the molecular architecture, concen-

tration, and environmental conditions. Figure 5 shows the

LCST versus degree of acetalysis for three APVA with

different Mw. The LCST of APVA depends on the molecular

weight of the starting PVA and the degree of acetalysis.

Higher LCSTs were observed for the APVA with a shorter

chain length and lower degree of acetalysis. The LCSTs

decrease continuously with the increase in the acetalysis

Figure 2. 13C-NMR (DMSO-d6) of APVA (a) and CAPVA (b) (Mw¼ 13,000–23,000).


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degree of APVA. At the same degree of acetalysis, the higher

the molecular weight of the starting PVA, the lower is the

phase transition temperature.

To visualize the changes in APVA chain conformation

during the phase transition, the atomic force microscope

(AFM) was used to image the actual physical states of

polymer chains at nanometer- or subnanometer-scale

resolution as a function of temperature. For AFM exper-

iments, a 0.1ml APVA (A28, Mw¼ 124,000, LCST is 33.88C)solution at 0.044 wt% was deposited on a freshly cleaned

silicon wafer and then dried at 20 or 608C. Figure 6a and b

show the representative AFM images of APVA films formed

Figure 4. Turbidity of a water solution containing 0.044 wt% PVA or APVA at

varying temperatures (Mw¼ 124,000–186,000).

Figure 3. The DEPT spectra of cationic PVA.


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at 60 and 208C in air and observed under contact mode. AFM

images reveal that the process of association of APVA chains

occurs at 608C. The A28 film dried at 608C, above its LCST,

has a flattened globular appearance in the AFM image. Its

grains were about 2–5mm in diameter and 50–200 nm in

height. On the other hand, the film dried at 208C, below the

LCST of A28, had a smooth and uniform surface. This result

suggests that the interaction betweenA28 chains andwater is

strong for the APVA molecule in its fully extended state at

208C, i.e. A28 is completely dissolved in water and forms a

uniformly smooth film as the solvent is evaporated. At 608C,the A28 chains collapse and tend to aggregate to form A28

chain globules. The starting PVA films were prepared by the

same procedure. Both the PVA films, which dried at 20 and

608C, have quite smooth and uniform surfaces, similar to

those shown in Fig. 6b. (The AFM images are not presented

here.)

Thermal sensitivity of APVA is attributed to the hydro-

phobic acetal rings being introduced in the PVA chains. It is

assumed that, in water, the remaining hydrophilic hydroxyl

groups are in a good solvent and form intermolecular

hydrogen bonds with water, while the hydrophobic acetal

rings are in a poor solvent and form cage-like structures

surrounding them at low temperatures to make the APVA

dissolve in water. The free APVA chains remain disordered

with random coils in solution that are in full hydration,

which is particularly important for the stability of the APVA

chains in water. Heating of the solution causes destruction of

the hydrogen bonds and the exposure of acetal rings. The

hydrophobic groups tend to stick together in order to reduce,

as much as possible, their surface of contact with water,

leading to the formation of hydrophobic aggregates or

hydrophobic association. This is corresponding to a morpho-

logical change from theAPVA copolymer random coils to the

intermolecular hydrophobic association. As the temperature

increases, the number and size of hydrophobic aggregates

grow, which leads to strong light scattering within the

turbidimeter. As a result, a large increase in turbidity is

observed. The higher the degree of acetalysis of APVA, the

more hydrophobic the groups become, and as a result, a

lower LCST temperature.

The relatively inexpensive and large-scale commercial

polymer, PVA, is provided with the property of thermal

sensitivity by partial acetalization. Furthermore, the phase

transition temperature of the polymer can be readily

controlled by the degree of acetalysis. The natural properties

of PVA, mentioned in Introduction, and an adjustable LCST,

make the APVA a very promising material for a variety of

applications, such as sensor, switch, and recordingmaterials.

The acetal rings make the PVA thermally sensitive as well

as dissolvable in cold water. DSC was used to reveal the

Figure 6. AFM images of silicon wafers in A28 solution at (a)

608C and (b) 208C (0.044 wt%, Mw¼ 124,000).

Figure 5. Dependence of the phase transition temperature on the degree of acet-

alysis of APVA at the concentration of 0.044 wt%.


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reasons why APVA is dissolved in water at room

temperature much more easily than PVA, which has a low

solubility at room temperature. Figure 7 shows the DSC

heating traces of PVA and APVA. As can be seen, the PVA

has only the crystal-melting peak (Tm) at 2158C, andA28 only

exhibits the glass transition temperature (Tg) at 988C. It

means that the starting PVA forms strong inter- and

intrachain hydrogen bonding and makes the polymer highly

crystalline. The Tg transition of PVA becomes very trivial

because the segments of PVA are frozen by the crystals. So

for the highly hydrolyzed PVA, a preparation temperature of

above 808C is required for complete dissolution in water in

an acceptable time to make sure that the inter- and

intrahydrogen bonding in the PVA chains are disrupted

by thermal energy. With the APVA, the acetal rings keep

APVA from forming crystals because they damage the orders

of molecular chain and decrease inter- and intrachain

hydrogen bonding. Consequently, APVA can be dissolved

in water at room temperature.

The effect of NaCl on the LCST behavior of(C)APVAAPVA is a linear nonionic polymer that is water-soluble at

low temperatures, but phase separates when the temperature

is raised above the LCST. Ionized APVA can also show

temperature sensitivity. Cationic charges are introduced

by grafting GTMAC onto the APVA chains in this study. But

the measurable LCST of CAPVA solution is reached only

when the polymer has a very small amount of cationic

groups and a large number of acetal rings or in NaCl

solution.

The effect of NaCl on the LCST of APVA and CAPVA

solutions is shown in Fig. 8. The addition of NaCl makes the

LCST of both polymers appear at a lower temperature. The

LCST of CAPVA is highly dependent on the NaCl

concentration at all experiment conditions. For C0.65A28,

the LCST decreases obviously even at a very low NaCl

concentration. On the contrary, the LCST of A28 remains

almost at the same temperaturewhen theNaCl concentration

is lower than 0.1M and the LCST begins to decrease quickly

after that. There is a critical point in the plot. The LCST of

C0.65A28 is higher than that of A28 before the critical NaCl

concentration, it is lower than that of A28 after that.

Figure 9 illustrates the dependence of LCST of three

CAPVAs, with the same amount of cationic groups but

different degrees of acetalysis, on the concentration of NaCl.

The LCSTs of three CAPVAs decrease at the same time with

the increase in NaCl concentration. However, CAPVA

containing a higher amount of cationic groups has a higher

LCST than others at all the experimental NaCl concen-

trations. If the amount of cationic group increases or the

degree of acetalysis decreases further, the LCST of CAPVA

will not be observed below 708C at a low NaCl concentration

and shifts to higher temperature at a high NaCl concen-

tration.

Figure 8. Dependence of LCSTof CAPVA and APVA on the concentration of NaCl

(Mw¼ 124,000–186,000, concentration of PVA¼ 0.044 wt%).

Figure 7. DSC heating traces of the starting PVA and APVA.


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The cationic group and the NaCl concentration insure the

variation of the LCST. The cationic group increases the

expansion of CAPVA chains by the electrostatic repulsive

force in salt-free or low NaCl solution and the hydrophobic

aggregations are hindered greatly. The role of NaCl is not

only to screen the ionic interaction of CAPVA, but also to

alter the hydration of APVA andCAPVA. First, the Cl� anion

tends to have a stronger interaction with water molecules

and polarizes them. Thus, some of the original hydrogen

bonding between the hydroxyl of (C)APVA and water is

destroyed by the Cl�. Second, the Cl� ion interferes with the

hydrophobic hydration of the macromolecule by increasing

the surface tension of the cavity surrounding the backbone

and the acetal ring.40 These effects are similar to increasing

the temperature and lead to the salting-out of the polymers,

thereby lowering the LCST.

The reason why the LCST of CAPVA is lower than that of

APVA in high NaCl solution is discussed here. CAPVA has

more hydrophobic groups, introduced by the backbone,

acetal ring, and GTMAC, than the APVAwhen they have the

same degree of acetalysis. The ionic repulsion interaction

dominates the LCST in salt-free or low NaCl solution;

therefore, the LCST of CAPVA is much higher than that of

APVA. But the amount of hydrophobic group controls the

LCST of (C)APVA in high NaCl solution where the ionic

repulsive force is totally screened.

All of the above results mean that the LCST of (C)APVA

aqueous solution could bemodulated easily by themolecular

weight, degree of acetalysis, amount of cations, and NaCl.

LbL assembling of (APVA/CAPVA) on rayonfiberAPVA and CAPVA are thermo-sensitive water-soluble

polymers. Their thermal sensitivity makes them attractive

for applications that demand ‘‘smart’’ material responses to

environmental stimuli. The current work is attempted to

assemble the APVA and CAPVA multilayers on cellulose

fiber to render the fiber with temperature sensitivity. APVA

normally is a neutral polymer but shows small negative

charges in a basic solution, as shown in Table 1, because of

the residual hydroxyl group. The negative charges decrease

with the increasing of acetal rings. CAPVA shows positive

charges in a neutral aqueous solution and its charge density

remains invariant in solutions of different pH values.

Rayon fiber was chosen in this research as a model

cellulose substrate because its surface ismore even or smooth

compared to those from conventional mechanical or

chemical wood pulps. (A13/C0.7A11)8 and (A23/

C0.7A14)8 multilayers were assembled on rayon fiber in

0.44 wt% APVA or CAPVA solution with 5mM NaCl and

pH¼ 9 at room temperature. After assembling, rayon was

immersed into D-D water at 608C for 10min and dried in

oven at 608C to observe the thermal sensitivity of the surface.

Changes in surface morphology of (A13/C0.7A11)8 multi-

layers are revealed by the AFM images, as shown in Fig. 10.

Many more particles appear on the heat-treated surface. An

(A23/C0.7A14)8 assembled surface (AFM images not shown)

Figure 9. LCST versus NaCl concentration for 0.044 wt% CAPVA with molecular

weight 124,00–186,000.

Table 1. Charge density of APVA and CAPVA

APVAa CAPVAb

SamplesCharge

density (meq/g) SamplesCharges

density (meq/g)

A13 �0.073 C0.70 0.32A23 �0.056 C0.65 0.22A28 �0.043 C0.44 0.18A35 �0.039 C0.26 0.12A41 �0.036 C0.07 0.032

aMeasured with the solution of [APVA]¼ 10mM, [NaCl]¼ 5mMand pH¼ 9.bMeasured with the solution of [CAPVA]¼ 10mM, [NaCl]¼ 5mMand pH¼ 5–6.


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presents the same thermal sensitivity. The roughness values

of heat-treated and heat-untreated surfaces listed in Table 2

further illustrate the behaviors, i.e. the roughness of the

heat-treated surface approximately doubled for rayon fiber

as substrate. In contrast, the roughness of control samples

without L-b-L assembly remained the same or similar.

The same assembling was also performed on a wafer as a

further proof for the thermally sensitive surface made from

APVA/CAPVA adsorption. TheAFM images and roughness

are presented in Fig. 11 and Table 2, respectively. The size

and the height of particles on the heat-treatedwafer aremuch

larger than those on the heat-untreated surfaces. The wafer

surface became rougher than the original one too.

We believe a surface-modified fiber can be obtained by the

multilayer assembly of APVA and CAPVA on fiber. CAPVA

was adsorbed on the rayon surface driven by electrostatic

interaction between the fiber and CAPVA. Then the APVA

and CAPVAwere alternately adsorbed onto the fiber to form

the multilayers.

CONCLUSIONS

We have shown here that novel thermally sensitive polymers

have been successfully synthesized by acetalyzing and/or

ionizing the commercially available PVA. The molecular

structures of acetalyzed PVA (APVA) and cationic APVA

(CAPVA) were characterized by NMR spectroscopy. The

effect of molecular weight, component and NaCl on the

phase transition of (C)APVA aqueous solution was inves-

tigated systematically. The phase transition is the result of

the introduction of the hydrophobic acetal rings onto the

PVA chains and can be changed easily.

(APVA/CAPVA)8 multilayers were assembled on rayon

fiber and silicon wafer to render the surfaces of substrates

temperature responsive. AFM images reveal that the

particles on the surface became larger after the material

was treated at 608C, and the roughness of the surfaces was

Figure 10. Flattened images of non heat-treated (a) and heat-treated (b) (A13/C0.7A11)8assembled multilayers on rayon fiber.

Figure 11. Flattened images of non heat-treated (a) and heat-treated (b) (A13/C0.7A11)8assembled multilayers on wafer.

Table 2. Roughness (Rq) of multilayers by AFM contact

mode (500 nm scale)

Fiber/nm Wafer/nm

Surface of substrate 208C 6.3 0.1608C 5.8 0.1

(A13/C0.7A11)8 208C 4.8 1.2608C 11.6 5.5

(A23/C0.7A14)8 208C 6.5 0.9608C 10.5 4.5


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also increased, thus providing direct evidence for the

thermo-sensitivity of the substrates assembled with

(APVA/CAPVA)8 multilayers.

The natural properties of PVA and the adjustable LCSTmake

(C)APVA to be a very promising material for a variety of

applications such as sensor, switch, and recording materials.

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Properties of a novel thermal sensitive polymer based on poly(vinyl alcohol) and its layer-by-layer...

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