基于T1-T2*弛豫相关的聚氨酯橡胶相态结构和动力学特征
Insights into the Phase Structure and Dynamics of Polyurethane Rubber Using T1-T2* Relaxation Correlation
通讯作者: *Tel: 010-89733248, Email:jguo7@cup.edu.cn.
收稿日期: 2022-07-11 网络出版日期: 2022-08-23
基金资助: |
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Corresponding authors: *Tel: 010-89733248, Email:jguo7@cup.edu.cn.
Received: 2022-07-11 Online: 2022-08-23
磁共振技术具有非破坏性、对高分子链运动敏感等特点,是一种能够在分子水平上表征高分子系统相态结构和动力学特征的常用技术.本文利用T1-T2*弛豫相关研究了聚氨酯橡胶的相态结构和分子动力学特征,并用高斯衰减和指数衰减模型分析了聚氨酯橡胶的T1-T2*数据.聚氨酯橡胶的T1-T2*谱显示了三种类型的信号:晶体氢组分的T2*最短,过渡相氢组分具有中等的T2*,非晶体氢组分的T2*最长;但这三种氢组分表现出相近的T1,且T1随着聚氨酯橡胶硬度的增加或温度的降低而逐渐降低.三种氢组分的磁共振信号强度随聚氨酯橡胶的硬度和温度的变化而变化.随着聚氨酯橡胶硬度的增加,晶体氢组分含量增加,非晶体氢组分和过渡相氢组分的含量降低;随着温度的增加,晶体氢组分含量减少,过渡相氢组分含量保持不变,而非晶体氢组分含量增加.另外,聚氨酯橡胶的硬软比随温度的升高而降低.这些结果表明T1-T2*弛豫相关可用于聚氨酯橡胶的相态结构和动力学评价.
关键词:
Nuclear magnetic resonance (NMR) is a non-destructive technique that can reveal the phase structure and dynamics of polymers at the molecular level. It is sensitive to polymer chain mobility and requires minimal special sample preparation. We investigated the phase structure and molecular dynamics of polyurethane rubber (PUR) based on the T1-T2* relaxation correlation spectra, and analyzed the T1-T2* data by multi modal decay. The T1-T2* spectra showed three types of signals: rigid 1H with the shortest T2* value, interphase 1H with an intermediate T2* value, and mobile-amorphous 1H with the longest T2* value. The three 1H components exhibit the similar T1 values in PUR, which decreased with increasing hardness or decreasing temperature. The integrals of these signals depend on the durometer hardness and temperature for PUR. They increase for the rigid phase but reduce for mobile-amorphous phase and interphase with an increase of PUR durometer hardness. The rigid 1H component decreased and the mobile-amorphous 1H component increased with increasing temperature, while the interphase 1H component remained constant. In addition, the hard/soft ratio decreased with increasing temperature for PUR systems. These results indicated that T1-T2* spectra can be used to characterize phase structure and dynamics of PUR.
Keywords:
本文引用格式
郭江峰, MACMILLAN Bryce, BALCOM Bruce.
GUO Jiangfeng.
引言
聚氨酯橡胶(Polyurethane Rubber,PUR)是一种工业上重要的高分子材料,已广泛应用于汽车工业、国防、食品和医疗保健等领域[1⇓⇓⇓⇓-6].PUR由硬段和软段嵌段共聚物组成[7]:硬段由二异氰酸酯和小分子扩链剂的反应产物组成;而软段由低聚物多元醇组成,例如聚酯、聚醚和聚丁二烯[8].硬段和软段之间存在显著的极性差异,硬段易于填充,并分布在软段中的微区.硬段和软段可以相互转换,这个过程也被称为微相分离.PUR的物理性质取决于微相分离的程度,而微相分离受其相态结构和动力学的影响.因此,研究PUR的相态结构和动力学有助于了解其物理性质,从而可以更有效地开展其工业应用.目前已有很多研究高分子材料相态结构和动力学的方法,包括热激电流法[9]、差示扫描量热法[10,11]、X射线散射法[12⇓-14]和磁共振方法[7,15⇓⇓⇓⇓-20].其中,磁共振方法具有非破坏性、无需特殊样品制备过程和对高分子链运动敏感等特点[21⇓⇓-24],是一种在分子水平上表征高分子材料相态系统结构和动力学的常用技术[25⇓-27].
上式中,γ为旋磁比,ΔB为非均匀场的宽度.T2*不仅与自旋-自旋弛豫有关,还受磁场的非均匀性影响[33].当T2很短时,T2*近似等于T2.
目前对于高分子材料的时间域磁共振测量与分析以单一弛豫参数T2(或T2*)、T1和T1ρ为主.与单一弛豫参数相比,二维弛豫相关谱可用于区分不同的组分,并能更准确地分析高分子系统的相态结构和分子动力学.在很大程度上,二维弛豫相关实验类似于二维磁共振波谱实验.在二维磁共振波谱中,采集频率随时间的演变数据,然后进行傅立叶变换获得强度与频率关系的频谱.而在二维弛豫相关实验中,采集信号幅度随时间的演变数据,然后进行拉普拉斯逆变换获得信号强度与弛豫时间的图谱[34,35].常用的二维磁共振测量(包括T1-T2、T2-D和T2-T2等)由于可以直观显示离散组分,并且可以用于定量评价,故在多孔介质分析中很有价值. 尽管如此,它们却不适用于T2较短的高分子材料的测量,主要原因是当T2较短时,其回波数据衰减较快,当前磁共振仪器的最短回波间隔无法满足短T2信号采集的要求.本研究团队提出了新的二维T1-T2*弛豫相关方法[36⇓-38],将其用于页岩和煤的测量,并评价了它们的结构和组分.新的二维T1-T2*弛豫相关方法是将传统二维弛豫相关方法扩展应用到固体和类固体系统.本文尝试利用二维T1-T2*弛豫相关方法研究高分子材料的相态结构和动力学特征.
本文介绍了研究高分子材料相态结构和动力学的磁共振实验装置;阐述了二维T1-T2*弛豫相关测量的原理;提出了利用非线性拟合和拉普拉斯逆变换结合的方法反演高斯和指数衰减组合的T1-T2*数据,来获取PUR的T1-T2*谱;分析了PUR弛豫特性与硬度和温度的关系,并基于此评价PUR相态结构和动力学特性.
1 实验部分
1.1 温控磁共振实验装置
为研究PUR的相态结构和动力学特征,本文选择了三种不同硬度(40A、80A和75D,分别代表中等软度、硬的和超级硬)的耐磨PUR作为研究对象,并搭建了如图1所示的温控磁共振实验测量装置.所有T1-T2*弛豫相关实验均在Tecmag公司Redstone谱仪上进行,采用的是空白时间为10 μs的鸟笼型射频(Radio Frequency,RF)探头.磁体的静磁场强度为2.4 T,共振频率为100 MHz.图1中磁体中心的物体代表待测样品,本文选用的PUR是长度为5.08 cm、半径为0.48 cm的圆柱体.测量样品附近的红色物体表示温度传感器,它采用的是从Omega工程公司购买的薄膜电阻温度探测器.该探测器可以快速响应温度变化,并向温度控制器提供反馈.加热器元件固定在空气通过的金属管道内,用于冷却样品的气体是由冷却器中的液氮蒸发的氮蒸汽.Omega温度控制器读取温度传感器数值,并根据设定的温度决定是否需要加热或冷却气流,其温度误差在0.1 ℃以内.
图1
图1
温控磁共振实验装置示意图
Fig. 1
Schematic of experimental apparatus for magnetic resonance measurements at different environment temperatures
1.2 二维T1-T2*弛豫相关测量
二维T1-T2*弛豫相关测量采用的序列由饱和恢复脉冲和一个90˚射频脉冲组成[36⇓-38],可以表示为:[(90˚)n-TR-90˚-acq],其中n表示饱和恢复序列中含有n个90˚脉冲,TR表示等待时间,acq表示开始采集FID数据.实验测量过程中,当温度传感器显示所设温度时,等待10 min使样品温度和环境温度一致后,再进行T1-T2*弛豫相关测量,测量参数设置如下:TR取100 μs~2.0 s之间对数等间隔的40个点;为了使采集的数据信噪比较高,设置64次信号平均;单个FID中相邻数据点之间的时间间隔为0.2 μs,数据点个数为4 096.90˚脉冲的宽度与样品和温度有关.表1中列出了本文执行的不同样品不同温度时的T1-T2*实验.
表1 二维T1-T2*弛豫相关实验
Table 1
PUR样品 | 温度T/℃ | 90˚脉冲的宽度/μs |
---|---|---|
40A | 0 | 12.0 |
80A | 0 | 11.9 |
75D | 0 | 11.3 |
75D | 20 | 10.9 |
75D | 40 | 10.6 |
75D | 60 | 9.8 |
75D | 80 | 9.9 |
1.3 二维T1-T2*数据处理
图2
图2
75D聚氨酯橡胶在T=0 ℃、TR=2.0 s时的FID信号及其拟合结果
Fig. 2
FID signal and the fitted results of 75D PUR, detected with recovery time of 2.0 s at T = 0 ℃
根据衰减快慢程度将FID数据分成两部分:前面部分包括高斯衰减和指数衰减信号,后面部分全是指数衰减信号.基于FID数据后面部分的信号幅度
式中,
式中,t代表FID数据的采集时刻.
利用整个采集阶段FID信号幅度
同样地,截取(4)式高斯衰减信号幅度
式中,
利用整个采集阶段FID信号幅度
通过上述(2)~(7)式的拟合,得到晶相组分T2*(即
二维T1-T2*数据
式中,
基于PUR的FID衰减模式,(8)式可以改写为:
式中,
(10)式中的方程都属于第一类Fredholm方程,因此,可以利用拉普拉斯逆变换方法分别求解每个方程,从而获得对应的T1-T2*谱[38],将两个T1-T2*谱相加即可得到总的T1-T2*谱.
2 结果与分析
2.1 不同硬度的PUR的相态结构特征
PUR的微观结构决定了其宏观物理力学性能.本文利用T1-T2*弛豫相关实验研究了PUR的微观结构.在T=0℃时,对三种不同硬度的PUR进行了T1-T2*弛豫相关实验.T1-T2*相关谱(图3)均显示三个主要信号,其他微弱信号可能是由噪声引起的,可以忽略.图3中色度条代表T1-T2*谱中不同颜色对应的信号强度(无单位).我们将从短到长的三个T2*信号分别定义为I、II和III.对于40A样品,信号I、II和III的积分强度分别为2.60×105(19.9%)、7.40×105(56.6%)和3.08×105(23.5%);对于80A样品,信号I、II和III的积分强度分别为4.03×105(30.9%)、6.81×105(52.2%)和2.20×105(16.9%);对于75D样品,信号I、II和III的积分强度分别为1.07×106(70.4%)、2.88×105(19.0%)和1.61×105(10.6%).40A、80A和75D样品的总信号强度分别为1.31×106、1.30×106和1.52×106.三种样品中各组分对应的1H信号的T1和T2*如表2所示.
图3
图3
不同硬度的聚氨酯橡胶在T = 0 ℃时的T1-T2*谱.(a) 40A;(b) 80A;(c) 75D
Fig. 3
T1-T2* spectra of PURs with different durometer hardness at T = 0 ℃. (a) 40A; (b) 80A; (c) 75D
表2 基于T1-T2*谱测量的40A、80A和75D聚氨酯橡胶T1、T2*及信号积分强度
Table 2
PUR样品 | 信号I | 信号II | 信号III | 总信号强度 | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
T1/ms | T2*/μs | 信号强度 | T1/ms | T2*/μs | 信号强度 | T1/ms | T2*/μs | 信号强度 | ||||
40A | 360 | 18 | 2.60×105 | 360 | 39 | 7.40×105 | 323 | 121 | 3.08×105 | 1.31×106 | ||
80A | 323 | 20 | 4.03×105 | 335 | 38 | 6.81×105 | 301 | 106 | 2.20×105 | 1.30×106 | ||
75D | 217 | 20 | 1.07×106 | 234 | 46 | 2.88×105 | 225 | 167 | 1.61×105 | 1.52×106 |
式中,
图4
图4
T = 0 ℃时,由T1-T2*谱计算的不同硬度的聚氨酯橡胶的硬软比
Fig. 4
Calculated HSRs of PURs with different durometer hardness based on T1-T2* spectra at T = 0 ℃
2.2 PUR的分子动力学特征
从前面结果可以看出,PUR中包含晶相、过渡相和非晶相三种相态结构.高分子材料的宏观特性除与相态结构有关外,也与微观动力学密切相关.随着周围环境温度的变化,PUR的晶相区、过渡相区和非晶相区之间可能存在相互转化现象,因此,研究PUR动力学特征对充分了解和应用PUR非常有价值.PUR的耐温区间为-30 ℃~90 ℃,本文另外选取了4个代表性的温度值20、40、60和80 ℃,对75D PUR进行了T1-T2*弛豫相关测量,以研究PUR分子动力学特征.不同温度下的T1-T2*谱如图5所示.与T = 0 ℃时75D PUR的T1-T2*相关谱类似,其他温度下T1-T2*谱也有三个主峰,即T2*从短到长的晶相、过渡相和非晶相峰.不同之处在于,各组分的信号强度和弛豫时间随温度变化而变化.根据T1-T2*谱计算的各个信号的T1、T2*、信号积分强度和总信号积分强度如表3所示.
图5
图5
不同温度下的75D聚氨酯橡胶的T1-T2*谱.(a) 20 ℃,(b) 40 ℃,(c) 60 ℃和(d) 80 ℃
Fig. 5
T1-T2* spectra of 75D PUR at different temperatures of (a) 20 ℃, (b) 40 ℃, (c) 60 ℃, and (d) 80 ℃
表3 基于T1-T2*谱计算的75D聚氨酯橡胶不同温度下T1、T2*及信号积分强度
Table 3
温度/℃ | 信号I | 信号II | 信号III | 总信号强度 | ||||||||
---|---|---|---|---|---|---|---|---|---|---|---|---|
T1/ms | T2*/μs | 信号强度 | T1/ms | T2*/μs | 信号强度 | T1/ms | T2*/μs | 信号强度 | ||||
0 | 217 | 20 | 1.07×106 | 234 | 46 | 2.88×105 | 225 | 167 | 1.61×105 | 1.52×106 | ||
20 | 217 | 20 | 8.81×105 | 217 | 51 | 3.06×105 | 217 | 202 | 2.69×105 | 1.46×106 | ||
40 | 242 | 21 | 7.34×105 | 242 | 50 | 2.83×105 | 242 | 219 | 3.71×105 | 1.39×106 | ||
60 | 280 | 21 | 6.04×105 | 290 | 52 | 2.62×105 | 280 | 245 | 4.38×105 | 1.30×106 | ||
80 | 355 | 21 | 4.98×105 | 355 | 55 | 2.46×105 | 355 | 258 | 4.69×105 | 1.21×106 |
从表3可以看出,75D PUR的三种氢组分在每个温度(0~80 ℃)下都表现出相近的T1,这是由于三种氢组分的快速自旋扩散导致的;T1随着温度的升高而增加,温度越高,氢组分的移动性越强,因此氢组分的T1时间更长.三种氢组分的T2*时间也随温度升高而增加.高分子体系中氢组分的T2*主要由偶极-偶极相互作用决定,低温时,分子的运动较慢,偶极-偶极相互作用强,局部场的强度增强;高温时,分子活动性增加,偶极-偶极之间的距离及角度发生快速变化,导致偶极间的取向被平均化,相互作用部分被抵消,因此随着温度升高,偶极-偶极相互作减弱,呈现出T2*时间增加.与信号I和II相比,信号III的T2*随温度变化更明显,表明温度对非晶相氢组分T2*的影响比晶相和过渡相更大.表3还显示不同温度下75D PUR中信号I和II的强度均随温度升高而降低,但信号III的强度随温度升高而增加;总信号强度随温度升高而减小,满足居里定律[42]中指出的磁化强度与温度呈反比的规律.
不同温度下75D PUR的T1-T2*谱中信号I、II和III的强度比例及硬软比如图6所示.图6(a)中随着温度升高,信号I的强度占比降低,信号II的强度占比基本保持不变,而信号III的强度占比增加.随着温度的升高,信号II的强度降低,但强度占比恒定,这是由于样品总的磁化强度随温度升高而降低造成的.信号I和信号III的强度占比变化表明,晶相氢组分随着温度的升高而减少,但非晶相氢组分随着温度的升高而增加.高温导致分子间和链间作用力减弱,氢组分的移动性增加.一些晶相氢组分被转化为更具移动性的氢组分.在晶相氢组分转变过程中,过渡相氢组分含量保持恒定,其原因是温度升高后,晶体氢组分在向过渡相氢组分转变过程中,相同含量的过渡相氢组分也向非晶相氢组分转变,从而整个过程中,过渡相氢组分含量处于一个动态平衡.图6(b)中75D PUR的HSR随温度升高而降低.HSR的降低表明晶相氢组分含量减少,而非晶相氢组分含量增加,从而表明聚氨酯橡胶的硬度与温度呈反比.
图6
图6
不同温度下75D PUR的(a)信号I(●)、II(■)和III(▲)强度比例和(b)硬软比
Fig. 6
(a) Integral intensity proportions of signal I (●), II (■), and III (▲) and (b) HSRs of 75D PUR at different temperatures
3 结论
本文通过利用二维T1-T2*弛豫相关方法测量不同温度下、不同硬度的PUR来研究其结构和分子动力学特征.PUR的FID数据由高斯衰减和两个指数衰减组成,表明PUR包含三种不同相态的氢组分结构,分别对应晶相、过渡相和非晶相三种氢组分.本文提出的针对含有高斯衰减和指数衰减的T1-T2*数据反演方法可以准确获得聚氨酯橡胶的T1-T2*谱.PUR的T1-T2*谱表明,相同温度下,由于快速自旋扩散,三种氢组分表现出相近的T1.随着PUR硬度的增加,晶相的信号强度比例逐渐增加,T1时间逐渐减小,但每种氢组分的T2*略有波动.由于强偶极耦合作用,晶相氢组分比非晶相氢组分具有更短的T2*.随着温度的升高,一些晶相氢组分转变为非晶相氢组分,因为高温导致分子间和链间作用力减弱,氢组分的移动性明显提高.在晶相氢组分转变过程中,过渡相氢组分含量保持恒定.HSR反映了PUR在不同硬度和温度下的相态结构变化和动力学.PUR的HSR随温度的升高而降低.以上结果表明T1-T2*弛豫相关方法可用于PUR的相态结构和动力学评价.该方法也可用于研究其他高分子材料.另外,虽然本文T1-T2*弛豫相关测量是在2.4 T的超导磁体上进行的,但它在低场台式永磁仪器上也可以应用.
利益冲突
无
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Determining the selective impregnation of waterlogged archaeological woods with poly (ethylene) glycols mixtures by differential scanning calorimetry
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X-Ray scattering reveals ion-induced microstructural changes during electrochemical gating of poly (3-Hexylthiophene)
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URL
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Highly controlled polymers and nanostructures are increasingly translated from the lab to the industry. Together with the industrialization of complex systems from renewable sources, a paradigm change in the processing of plastics and rubbers is underway, requiring a new generation of analytical tools. Here, we present the recent developments in time domain NMR (TD-NMR), starting with an introduction of the methods. Several examples illustrate the new take on traditional issues like the measurement of crosslink density in vulcanized rubber or the monitoring of crystallization kinetics, as well as the unique information that can be extracted from multiphase, nanophase and composite materials. Generally, TD-NMR is capable of determining structural parameters that are in agreement with other techniques and with the final macroscopic properties of industrial interest, as well as reveal details on the local homogeneity that are difficult to obtain otherwise. Considering its moderate technical and space requirements of performing, TD-NMR is a good candidate for assisting product and process development in several applications throughout the rubber, plastics, composites and adhesives industry.
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核磁共振多回波串联合反演方法
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PMID:26404771
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We review basic principles of low-resolution proton NMR spin diffusion experiments, relying on mobility differences in nm-sized phases of inhomogeneous organic materials such as block-co- or semicrystalline polymers. They are of use for estimates of domain sizes and insights into nanometric dynamic inhomogeneities. Experimental procedures and limitations of mobility-based signal decomposition/filtering prior to spin diffusion are addressed on the example of as yet unpublished data on semicrystalline poly(ϵ-caprolactone), PCL. Specifically, we discuss technical aspects of the quantitative, dead-time free detection of rigid-domain signals by aid of the magic-sandwich echo (MSE), and magic-and-polarization-echo (MAPE) and double-quantum (DQ) magnetization filters to select rigid and mobile components, respectively. Such filters are of general use in reliable fitting approaches for phase composition determinations. Spin diffusion studies at low field using benchtop instruments are challenged by rather short (1)H T1 relaxation times, which calls for simulation-based analyses. Applying these, in combination with domain sizes as determined by small-angle X-ray scattering, we have determined spin diffusion coefficients D for PCL (0.34, 0.19 and 0.032nm(2)/ms for crystalline, interphase and amorphous parts, respectively). We further address thermal-history effects related to secondary crystallization. Finally, the state of knowledge concerning the connection between D values determined locally at the atomic level, using (13)C detection and CP- or REDOR-based "(1)H hole burning" procedures, and those obtained by calibration experiments, is summarized. Specifically, the non-trivial dependence of D on the magic-angle spinning (MAS) frequency, with a minimum under static and a local maximum under moderate-MAS conditions, is highlighted. Copyright © 2015 Elsevier Inc. All rights reserved.
Properties of polyurethane elastomers with different hard/soft segment ratio
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Curie law for Anderson's model of a dilute alloy
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