Characterization of water state and distribution in fibre materials by low-field nuclear magnetic resonance

Abstract

The water state and distribution in PET and cotton fibres were studied by low-field proton nuclear magnetic resonance. The spin–spin relaxation times (T₂) were measured with single pulse free induction decay (FID). There are three different states of adsorbed water in the fibre materials. The slowest fraction T₂₂ can be assigned to the bulk water. The intermediate component T₂₁ can be ascribed to microporous structure confined water. The fastest fraction, T_2b, can be assigned to the water molecules trapped by hydrogen bond owing to the chemical group. During desorption process of fibre materials, three types of water in the fibre materials work together to present the time-domain spectra, where three peaks are really related each other. Based on the interaction relationship between multi-structure of fibre materials and adsorbed water, PET fibre materials were chosen to investigate the adsorption and desorption behavior designed by copolymerization and morphology design method. The experiments of surface contact angle of fibre and fabric, moisture adsorption, water adsorption, wicking distance and water vapor permeability were carried out. The results show that the designed PET fibre materials have fast adsorption–desorption capacity. LF-NMR provides unique insight into the water state and distribution of multi-structure of fibre materials.

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Introduction

An important function of textiles is supplying the maximum wearing comfort to human bodies.¹ Clothing materials should thus have a high water retention capacity and high water transportation properties to maintain a constant temperature and humidity between skin and fabric when the human body is in a state of movement or hot environment.^2,3 When fibre materials are in service, water can be adsorbed to the materials and the interaction between water and materials has large effects on their physical properties.^4–6 At the same time, the water adsorbed to the materials exhibits a different structure from bulk water.⁷ Hence, understanding the mechanism of water sorption on fibre materials may provide hints to improve not only the wearing comfort but also their textile processing.⁸

Various theories have been proposed to describe the sorption mechanisms of fibre materials. Peirce introduced a model which is based on the assumption of direct and indirect sorption of water molecules on attractive groups of the materials.⁹ A theory in which the interaction between water and the binding sites considers three types of water with different associating strength was proposed.¹⁰ Langmuir developed the classical model for adsorption isotherms.¹¹ Young and Nelson developed a complete sorption–desorption theory, starting from the assumption of a distinct behavior of bound and condensed water (Young & Nelson).¹² These theories have been promoted the development of fibre material, but we also find that there less experimental characterization for water state and distribution in fibre materials.^13,14

Interactions between fibre materials and water have been considered to play an important part.^15,16 Therefore, the interactions have long been a matter of extensive studies.^17–20 Many efforts have been made to investigate the states of water in the matrix by various methods, including differential scanning calorimetry (DSC),²¹ Fourier transform infrared spectroscopy (FTIR),²² NMR,²³ X-ray adsorption spectra²⁴ etc. However, there is a clear difference of measurement results between these methods.²⁵ For example, it is not straightforward to interpret NIR spectra because the spectra are often very complicated owing to the overlapping of a number of overtones and combination bands in the NIR region.²⁶ The difficulty of monitoring the water state and distribution in fibre material lies in the fact that fibre materials have complex structure, including a combination of chemical, morphological structure.²⁷ Addition, most off-line measurement that typically used to monitor water state and distribution are not effective for real-time monitoring and control purposes due to a large delay.^28–30

As NMR spectroscopy reveals information about molecular structure and mobility, it became one of the most powerful non-destructive analytical tools by measuring the protons spin–spin relaxation time T₂.³¹ The terms low field, intermediate field, and high field do not carry any strict meaning.

High-resolution NMR spectroscopy provides a method of structural determination for complicated molecules at magnetic field at least of 7.05 T, corresponding to 300 MHz spectrometer.³² Low-field NMR systems equipped with permanent magnets are available for quantitative analysis in quality control, also as on-line instruments in production environments at magnetic field strengths up to about 1 T, corresponding to 42.5 MHz ¹H-frequency.³³

In the recent decade, low-field nuclear magnetic resonance is attracting attention as a rapid, nondestructive and low-cost technique in the materials science.^34,35 The state and distribution of water in polymers can be determined by LF-NMR method while high-resolution NMR may destroy the mobility of water absorbed in polymer because of higher frequency pulse.³⁶ The state and distribution of water protons in matrix are well reflected by LF-NMR. Because of the different microenvironments that the protons experience, LF-NMR can also be regard as powerful tool to study the microstructural properties of materials matrix.³⁷ In general, the relaxation time of complex systems exhibits multi-component behavior, in which each component can be interpreted as representing different water states by LF-NMR method. LF-NMR measurements are successfully applied in the quality control of various types such as the water mobility and distribution in poly(ethylene glycol)³⁸ and poly(sulfobetaine methacrylate).³⁹

In the present study, low-field nuclear magnetic resonance was introduced for the elucidation of water state and distribution in PET and cotton fibre materials. Based on the different relaxation time of water in fibre, the water state was proposed and the change of water state was monitored during desorption process. The water adsorption and desorption kinetics in PET fibre materials designed by copolymerization and morphology design method was investigated. The results attempt to address the water adsorption and desorption mechanism of materials with complicated structure and guide the preparation of possibility of hydrophilic PET fibre materials by LF-NMR method.

Experimental

Materials

To distinguish between exchanging protons and non-exchanging protons, deuterium oxide (D₂O) experiments were performed. Deuterated water (D₂O) and manganese chloride used in experiments were purchased from J&K Chemical. All materials were used without further purification.

Preparation of PETG copolymers and fibres

PETG copolymers were synthesized by macromolecular interesterification method. A representative polymerization procedure was as follows. For the esterification process, the temperature was controlled at 245 °C. The time of esterification process was about 3 hours. After which the complete removal of the anticipated produced water took place, PEG (10 wt% relative to PTA) was added into the reactor, together with antimony as ethylene glycol catalyst. Polycondensation reaction occurred at the temperature 270–275 °C for 2–2.5 h.

The copolymers were designed as PETG800, PETG2000, PETG6000 and PETG20000, respectively. The samples were kept in an vacuum oven at 105 °C for 48 h. PETG copolymers were molten spinning to get the PETG fibres.

Preparation of fabrics

One hundred percent fibres 22.7 tex combed yarn was used to manufacture the fabric. The fabric was knitted in single jersey structure of 160 grams per square meter (gsm) at Jinhui New Fibre Materials Company (Suzhou, China). The fabric has the following geometrical properties: course per inch (CPI) = 48, wales per inch (WPI) = 37, stitch length = 2.75 mm. The color of the grey cotton fabric was creamy white.

Alkali reduction experiment

To investigate the relaxation time of adsorbed water by microporous structure of fibre materials, alkali reduction experiment of PET fibre materials was carried out. Cleaned and dried PET fibre dipped in 0.1 mol L⁻¹, 0.2 mol L⁻¹, 0.4 mol L⁻¹ and 0.8 mol L⁻¹ NaOH solution in 30 min at 90 °C, respectively. The bath ratio for fibres to solution was 1 : 50. After alkali reduction treatment, the fibres were washed 5 times with deionized water, and then dried in the oven at 105 °C for 4 h.

Characterization methods

Measurement of low field nuclear magnetic resonance. A low field pulsed NMI 20-Analyst (Resonance Instruments, Shanghai Niumag Corporation China) with 18.4 MHz was used in the experiment. Strip sample was placed in a 25 mm glass tube and inserted in the NMR probe. Carr–Purcell–Meiboom–Gill (CPMG) sequences were employed to measure spin–spin relaxation time, T₂. The maximum point of every second echo was accumulated using a 90° pulse of 17 μs and a 180° pulse of 34 μs. The delay between the 180° pulses, τ, was 500 μs. The amplitude of every second echo was measured, a total of 18 000 points were collected, and scans were coded using a repetition time (TR) of 4 s. The relaxation measurements were performed at 32 °C.

MultiExp Inv Analysis software (Niumag Co., Ltd, Shanghai, China) was employed for data analysis. This software performs distributed exponential curve fitting. In the time domain, spin–spin relaxation data is presumed to be a sum of exponentials:

I_CPMG(t) = ∑I_m exp(−t/T_2m) (1)

T_2m is the relaxation times of the less mobile (said ‘‘solid”) and the mobile populations respectively. I_m is the intensity of the mobile populations. The curves were then deconvoluted to Gaussian functions (each population being represented by a Gaussian) for a better schematic representation of the T₂ spectra. The intensity of peaks has been as the relative to the samples (g).

Moisture adsorption capacity. 50 g of fibre materials were kept at the condition of 20 °C and relatively humidity 65%, and then weighted the fibre (W₁). The sample was dried at 105 °C for 1 h and the sample was reweighed (W₂). The moisture adsorption (WA) was calculated from eqn (2).

WA (%) = (W₁ − W₂)/W₂ × 100% (2)

Water adsorption capacity. 0.5 g of fibre was soaked into distilled water for 24 hours at ambient temperature. After centrifugation at 4000g for 10 min, the weight of the fibre was measured (W₃). The sample was dried at 105 °C for 4 h and the sample was reweighed (W₄). The water retention value (WRV) was calculated from eqn (3).

WRV (%) = (W₃ − W₄)/W₄ × 100% (3)

Surface contact angle of fibres and fabrics. The experimental procedure of surface contact angle of fibres is as follows: after being selected, the fibres were cleaned. Fibres that possess diameters between 50 μm were used. A sessile drop of water with a volume of 10 μL was vertically deposited on the sample using the equipped syringe pump and the static water CA was recorded. Then pure water was continuously and slowly pumped into the droplet at a speed of 5 μL min⁻¹ until the droplet reached 20 μL, and the evolution of the water droplet was simultaneously recorded twice a second with apparent CAs calculated.

The water contact angles of fabrics were as follow: a drop of distilled water (2 μL) was placed at a rate of 0.5 μL s⁻¹ by a micro syringe. Images of the drop were recorded up to 10 s after the drop was initially set on the copolymer surface. The analysis was performed on ten different spots of the sample.

Scanning electron microscopy. SEM (Hitachi SU8010) was used to observe the microstructure of the fibres after treatment of alkali reduction experiment.

Wicking distance and diffusion time characterization of fabrics. The prepared fabrics were made in the size of 2 cm × 20 cm and one end of samples were immersed in water for 30 min to meter the height for wicking distance; the prepared fabrics were made in the size of 10 cm × 10 cm and 0.5 mL water was dropped onto the samples, then calculated the time that water spread out completely on the fabrics.

Results and discussion

Relaxation characteristic spectrum of free water

To discuss the water mobility and distribution of fibre material/H₂O system, it is necessary for us to investigate the properties of free water without restricted effect. LF-NMR was performed by applying a radio frequency pulse to a sample within a magnetic field and assessing the relaxation properties of the proton of the sample. The inversion spectra of the T₂ of free water show only one peak, which occur at the T₂ of 2848.1 ms (shown in the Fig. 1). The protons of free water are observed in only one status since it is a homogeneous single content.

Fig. 1 T₂ inversion spectra (signal intensity versus corresponding relaxation time) for varied amounts of H₂O from 1.056 g to 7.123 g. The signal intensity represents the number of protons in the corresponding relaxation time.

The integrated signal intensity of the inversion spectra versus varied amounts of water is shown in Fig. 2. The integrated signal intensity is linear with the amount of water and can directly represent all the proton numbers in each sample. The degree of fitting is 0.999, indicating that the inversion algorithm for T₂ is very reliable and accurate.

Fig. 2 The integrated signal intensity of water peak in the inversion spectrum versus the amount of H₂O.

Water mobility and distribution in fibre material/H₂O

In order to detect the mobility change of fibres mixtured with amount of water and exclude the pure water trials, D₂O was used. It is clear that there are three new protons status occurring for PET and cotton fibres mixtured with H₂O (Fig. 3) compared with D₂O (Fig. 4). The peaks belong to the water protons since when H₂O is replaced by D₂O. The relaxation times of three kinds of water are in the order of T₂₂ > T₂₁ > T_2b. As shown in Fig. 3, T_2b was the fastest fraction, with a relaxation time of 10 ms and accounted for more than 5% of the total signal. T₂₁ was the intermediate fraction (approximately 5% of the total signal) with a relaxation time of approximately 100 ms, whereas T₂₂ was the slowest and had the maximum fraction, with a relaxation time of approximately 1000 ms (more than 90% of the total signal). There is a small peak occurring in Fig. 4, which mainly comes from the small amount of H₂O left in the fibre samples. The interpretation and origin of the T₂ transverse relaxation of heterogeneous systems have been studied extensively.^40,41 In general, multiexponential relaxation behavior of various materials matrixes can be ascribed to different water fractions that exist in heterogeneous microstructures and protons of matrix.

Fig. 3 T₂ inversion spectra (signal intensity versus corresponding relaxation time) for H₂O/fibre mixtures after centrifugation at 4000g for 10 min.

Fig. 4 T₂ inversion spectra (signal intensity versus corresponding relaxation time) for D₂O/fibre mixtures after centrifugation at 4000g for 10 min.

On the basis of the fibre material/H₂O experiment, it is reasonable to propose the origin of three proton fractions are assigned to the water molecules. Previous studies have shown that protons in fibre material systems can be differentiated into several distinct populations through their NMR relaxation behavior, which provides unique insight into the multiple structures of the fibre materials. The highest relaxation time corresponded to the most mobile population. The slowest fraction T₂₂ can be assigned to the bulk water implying the T₂₂ water was located in a more mobile environment. The intermediate component T₂₁ can be ascribed to microporous structure confined water. The fastest fraction, T_2b, can be assigned to the water molecules trapped by hydrogen bond owing to the chemical group such as the –OH. To further investigate the relationship between the structures of fibre materials and relaxation behaviors and validate this assumption of relaxation times owing to the multiple structures, the T₂ of adsorption water by chemical group and microporous structure were measured. Fig. 5 shows the relaxation times of different chemical group, including the –OH, –SO₃–, –NHCO– and –COOH. We observed there is only one peak in chemical group/H₂O system although there is difference of relaxation time among these at the same mole of chemical group to water. The results of relaxation time are shown in Table 1. Note that T₂ (10–100 ms), this population is explained by the reason that the water is adsorped by hydrogen bond of chemical group. Due to the lack of hydrophilic adsorption group in PET fibre materials, the population of T_2b is less than that of cotton fibres.

Fig. 5 The relaxation times of different chemical group at the same mole of chemical group to water.

Table 1 The relaxation time of chemical group by LF-NMR method^a

Group	Molar ratio of H2O to group	T2′ (ms)	T2′′ (ms)	T2 (ms)
a T2′, T2 and T2′′ represent the start, peak and end time, respectively.
–OH	5 : 1	1.52	14.17	4.64
–SO3Na	5 : 1	4.02	265.61	65.79
–COOH	5 : 1	57.22	75.65	65.79
–NHCO–	5 : 1	65.79	151.99	86.97

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To understand the localization of T₂₁, it is informative to know the microstructure of fibre materials. The influence of morphological structure of fibre material on water state and distribution will be discussed. Compared with the populations of T₂₁, there is a distinct difference between PET and cotton fibre materials. The populations of T₂₁ may be assigned to water adsorbed by the surface microstructure of fibre material. The smooth surface of PET fibre material leads the signal intensity of T₂₁ less than that of cotton fibre materials. Here, to explore the relaxation time of adsorbed water by microporous structure of fibre materials, alkali reduction experiment of PET fibre materials was carried out. Fig. 6 shows the surface morphology of PET fibre materials under the condition of alkali reduction with different alkali concentration. It is clear to observe that with the increase of alkali concentration, the microporous structure of fibre materials is gradually distinct, especially at the higher alkali concentration. The PET fibre materials samples after alkali reduction treatment were used to evaluate the influence of microporous structure of fibre materials on the relaxation time of adsorbed water. Fig. 7 shows the population of T₂₁ of PET fibre materials after alkali reduction treatment with different alkali concentration. Compared with the population of T₂₁, there is no obvious change of the signal intensity of T_2b and T₂₂, indicating the microporous structure of fibre materials has great effect on the population of T₂₁. With the treatment of alkali concentration increasing, the signal intensity of T₂₁ increases, according to the influence of alkali concentration on the number of microporous on the surface of PET fibre materials, especially at the higher alkali concentration. It can prove that the signal intensity of T₂₁ is attributed to microporous on the surface of fibre materials. From Fig. 7, we can also get the information that with treatment of alkali concentration increasing, the relaxation time T₂₁ of water adsorbed by microporous increases.

Fig. 6 The surface morphology of PET fibre under the condition of alkali reduction with different alkali concentration. (a) 0.1 mol L⁻¹; (b) 0.2 mol L⁻¹; (c) 0.4 mol L⁻¹; (d) 0.8 mol L⁻¹.

Fig. 7 The population of T₂₁ of PET fibre materials after alkali reduction treatment with different alkali concentration. (a) 0.1 mol L⁻¹; (b) 0.2 mol L⁻¹; (c) 0.4 mol L⁻¹; (d) 0.8 mol L⁻¹.

Based on the analysis of T_2b and T₂₁, we find that T₂₂ was close to that of bulk water (free water). The slowest exchangeable and highest mobile fraction, T₂₂ was ascribed to mobile bulk water existing between fibres and fibres. The relaxation time of multiple structures reflects the state of adsorbed water in the fibre materials. We can conclude the order of interaction between multiple structures and water at T′_2b > T′₂₁ > T′₂₂. The interaction between adsorbed water and fibre materials has large effects on its adsorption and desorption properties.

The wearing comfort of fabrics mostly lies in its adsorption and desorption properties. It is necessary for us to further investigate the influence of mobility of water adsorbed by multiple structures of fibre materials on the adsorption or desorption process.

Water state and distribution during fibre materials desorption process

There are three different water states in the cotton and PET fibre materials from above study (shown in Fig. 8). It is also worthy for us to investigate the mechanism of water state and distribution during desorption process of the fibre materials. The signal amplitude of different states of adsorbed water in fibre materials during water desorption process are shown in Fig. 9, T_2b, T₂₁ and T₂₂ decreased. During desorption process of cotton fibre or PET fibre materials, it is most easily for bulk water (T₂₂) to desorb from fibre materials matrix. When the bulk water amount decrease, T₂₁ becomes much shorter, since intermediate water interact with water hydrogen bonding with or the hydroxyl sites on polymer surface.⁴² The decrease of T₂₁ of absorbed water by microporous structure also has great effect on T_2b adsorbed by hydrogen bond, leading the decrease of T_2b.⁴³ The relaxation time further indicates the mechanism of water in fibre materials during desorption process. Three types of water in the fibre materials work together to present the time-domain spectra, where three peaks are really related each other. During desorption process of fibre materials, it is possible for the microstructure of fibre materials and water in the system to change. When the water is adsorbed by fibre materials, the mobility of water could chang. On the other hand, the interaction between water and fibre materials could have great influence on the property of fibre materials, such as swelling phenomenon.⁴⁴

Fig. 8 The states of absorbed water in the cotton and PET fibres materials characterized by LF-NMR method.

Fig. 9 The signal amplitude and relaxation time of different adsorbed water in fibre materials during desorption process. (a) PET fibre materials; (b) cotton fibre materials.

LF-NMR provides unique insight into the water state and distribution during water desorption process. Above all the mechanism of the state and distribution of adsorbed water in the fibre materials, the relationship between the relaxations time (interaction) and desorption capacity was provided. Table 2 shows the measurement of relaxation time and desorption capacity of water adsorbed by chemical group, microporous and bulk water. The relaxation times 1–50 ms, 100–500 ms, 500–1500 ms of water adsorbed by chemical group, microporous structure and bulk water respectively. The order of desorption capacity of multiple structures is at T₂₂ > T₂₁ > T_2b. The results can provide the information that helps us to construct the ideal comfort clothing. Ideal clothing materials should thus have a high water retention capacity and high water transportation properties to maintain a constant temperature and humidity between skin and fabric when the human body is in a state of movement or hot environment.

Table 2 The relaxation time of interaction of fibre materials

Adsorption structure	Adsorbed water	Interaction (kJ mol^−1)	T2 (ms)
Chemical group	T2b	12.6–29.3	1–50
Microporous structure	T21	4.2–8.4	100–500
Bulk water	T22	<2	500–1500

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The construction of fast adsorption and desorption of fibre materials

Comfortable clothe always absorbs moisture from atmospheres of greater humidity and releases it to a drier environment so as to create a balance in moisture conditions. In summer it is hot, and the fabrics for clothing are normally light, thin, and ventilated to make the wearer feel cool and comfortable. The human body tends to sweat, causing a moisture gradient from the human skin to the surrounding outdoor environment.

Based on the relationship between the interactions and desorption capacity, the fibre materials should have good wetting behavior and water retention capacity (water adsorbed at T₂₁ and T₂₂) to reach the goal of fast adsorption–desorption process between fabrics and skin. Thus the design and study of comfortable fibre materials with liquid water moisture management properties is necessary to meet the requirements under these circumstances. Cotton fibre has higher water absorbency (water adsorbed at T_2b and T₂₁) than synthetic fibres such as PET fibre while the desorption capacity of PET fibre is better than that of cotton fibre. In this experiment PET fibre materials were chosen to further study. Firstly, PEG was used the chemical copolymerization modifier to improve the PET fibre wetting behavior. PETG copolymers with PEG of molecular weight from 800 to 20 000 g mol⁻¹ were synthesized by melt macromolecular interesterification method and then the PETG copolymers were molten spinning to get the PETG fibres. Fig. 10 shows the surface wetting behavior of PETG fibres. With the molecular weight of PEG increasing, the wetting behavior of PETG fibres increased. PETG fibre with the molecular weight 2000 g mol⁻¹ of PEG was used to the next experiment. The hydrophilic property results of fibre materials reflects the nature of materials, but for comfortable clothes, comprehensive properties of fabric should be as the object to be further considered. It is necessary for us to investigate the fabrics designed by these fibre materials.

Fig. 10 The surface wetting behavior of PETG fibres with different molecular weight of PEG from 200–8000 g mol⁻¹.

Fig. 11 shows the surface wetting behavior of PETG (shown in Fig. 11a) and PET fabrics (shown in Fig. 11b). When the water drop contacts with PETG fabrics, it can be immediately adsorbed into fabrics and the result of surface contact angle is about zero. At the condition of same designed structure of fabrics, PETG fabrics shows greater wetting behaviors compared with that of PET fabrics. Compared with untreated of PET fabrics, the wetting behavior of PET after treatment of hydrophilic finishing agent is more superior. The treatment of hydrophilic finishing agent of fabrics has been as the most acceptable method to improve water adsorption capacity. But this method has poor wash resistance that the surface hydrophilic property decreased significantly due to the loss of finishing agent.^45,46

Fig. 11 The surface wetting behavior of fabrics. (a) PETG fabrics; (b) PET fabrics; (c) PET fabrics after treatment of hydrophilic finishing agent.

The surface wetting behavior of fabrics reaches the first step of fast adsorption–desorption process, the water adsorption capacity of fabrics will be further studied. The states of adsorbed water in the fibre materials has been investigated above. The water content adsorbed by microporous structure should be increased to satisfy a high water retention capacity to maintain a constant temperature and humidity between skin and fabric when the human body is in a state of movement or hot environment. The PETG and PET fibres were designed to the cross shaped structure to have a microstructure. The signal amplitude and relaxation time of different adsorbed water in fibre with cross shaped structure is shown in Fig. 12. The cross-structure of PET and PETG fibre materials leads the T₂₂ water increase significantly due to the microstructure adsorption capacity. Besides that the T_2b bound water content of PETG fabrics is higher than that of PET fabrics. This is caused the introduction of chemical group –O– of PEG into molecule, improving the number of hydrogen bond.

Fig. 12 The signal amplitude and relaxation time of different adsorbed water in fibres with cross-shaped structure. (a) PET fabrics; (b) PETG fabrics designed by cross-shaped structure fibre materials; (c) PET fabrics designed by cross-shaped structure fibre materials.

Table 3 shows the comprehensive evaluation adsorption and desorption properties of fabrics, including the moisture adsorption (MA), water adsorption capacity (WAC), wicking distance, diffusion time. The results of MA and WAC measurement reflect the hydrophilic property and adsorption capacity. There is no difference of WAC between PET, PETG and PET after treatment due to microstructure adsorbed water and free water account for the major adsorbed water. The moisture adsorption of PETG fabrics is higher than that of PET because of hydrophilic group –O– in the fibre materials. The experiment date of wicking distance and diffusion time describes the water transport behavior. Though PET fabric after treatment has excellent water transport behavior, it decreases obviously after water wash 10 times. The water vapor permeability illuminates the water adsorbed by multi-structure fabrics the desorption property. PETG fabrics retains the fast desorption advantage of PET. It may be ascribed to the design of multi-structure of fibre materials and control of interaction between multi-structure and water.

Table 3 The water adsorption and desorption date of PET and PETG fabrics

Samples	MA (%)	WAC (%)	Wicking distance (cm)	Diffusion time (s)
a Represents the fabrics after water wash 10 times.
PETG	1.2	125	24	<0.5
PET	0.4	120	20	>3.0
a-PET	0.5	120	22	1.8
PETG^a	1.2	125	24	<0.5
PET^a	0.4	120	20	>3.0
a-PET^a	0.4	120	20	>3.0

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Conclusions

There are three different states of adsorbed water in the fibre materials. The interaction between multiple structures and water is at the order of T_2b > T₂₁ > T₂₂. The analysis of T₂ component with different states shows that there are three categories of water. The slowest fraction T₂₂ can be assigned to the bulk water. The intermediate component T₂₁ can be ascribed to microporous structure confined water. The fastest fraction, T_2b, can be assigned to the water molecules trapped by hydrogen bond owing to the chemical group. When the water is adsorbed by fibre materials, the mobility of water could change. On the other hand, the interaction between water and fibre materials could have great influence on the property of fibre materials. Three types of water in the fibre materials work together to present the time-domain spectra, where three peaks are really related each other. Based on the interaction between multi-structure and water, PET fibre materials were chosen to investigate the adsorption and desorption behaviors designed by copolymerization and cross-shaped method. The experiments of surface contact angle of fibre and fabric, moisture adsorption, water adsorption, wicking distance and water vapor permeability were carried out. The results show that the designed PETG fabrics have fast adsorption–desorption capacity. LF-NMR provides unique insight into the water state and distribution of multi-structure of fibre materials.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (61134009), National Basic Research Program of China (2011BAE05B00, 2013BAE01B02), and the Program for Innovative Research Team in University of Ministry of Education of China (IRT1221). We also think the technical support staff from Shanghai Niumag Corporation (Shanghai, China).

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